Feature: New Downloader feature.

Use this feature to prepare tick data before using ticks or stream API.

Use metadata API for all API.
This ensures valid instrument used from the start.

README.md

@@ -1,18 +1,20 @@
 ## 1 Tick Data Downloader
 
-- Download tick data from [Dukascopy](https://www.dukascopy.com/swiss/english/marketwatch/historical/) 
+- Download tick data from [Dukascopy](https://www.dukascopy.com/swiss/english/marketwatch/historical/)
 - Convert source tick data to CSV/HST/FXT
 
 ### 1.1 Building
 
 To build, you need to install go (https://golang.org/)
 
 To build:
+
 ```
 go build
 ```
 
 ### 1.2 Running
+
 Get January (2019) data for EURUSD and convert to FXT/MT4
 
 ```
@@ -26,7 +28,6 @@ Get 2018 data for XAUUSD and convert to FXT, and then to HST
 ./go-duka -symbol EURUSD -format hst -start "2018-01-01" -end "2018-12-31"
 ```
 
-
 ## 2 CSV Format
 
 #### 2.1 Example
@@ -63,7 +64,7 @@ type Header struct {
 #### 3.2 Bar Data
 
 - Total Bytes : 60
-- **CTM** : Current time in seconds, align with timeframe, 
+- **CTM** : Current time in seconds, align with timeframe,
 
 ``` Golang
 // BarData wrap the bar data inside hst (60 Bytes)
@@ -89,14 +90,14 @@ type BarData struct {
 ```Golang
 // FxtTick data
 type FxtTick struct {
-	BarTimestamp  uint64  //   0  8   Bar datetime, align with timeframe, unit seconds
-	Open          float64 //   8  8
-	High          float64 //  16  8
-	Low           float64 //  24  8
-	Close         float64 //  32  8
-	Volume        uint64  //  40  8   Volume (documentation says it's a double, though it's stored as a long int)
-	TickTimestamp uint32  //  48  4   tick data timestamp in seconds
-	LaunchExpert  uint32  //  52  4   Flag to launch an expert (0 - bar will be modified, but the expert will not be launched).
+BarTimestamp  uint64  //   0  8   Bar datetime, align with timeframe, unit seconds
+Open          float64 //   8  8
+High          float64 //  16  8
+Low           float64 //  24  8
+Close         float64 //  32  8
+Volume        uint64  //  40  8   Volume (documentation says it's a double, though it's stored as a long int)
+TickTimestamp uint32  //  48  4   tick data timestamp in seconds
+LaunchExpert  uint32  //  52  4   Flag to launch an expert (0 - bar will be modified, but the expert will not be launched).
 }
 ```
 
@@ -105,12 +106,11 @@ type FxtTick struct {
 - Total Bytes : 728
 - **Version** : 405 default
 - **Symbol** : forex currency pair
-- **Spread** : fix spread 
+- **Spread** : fix spread
 - **Period** : timeframe, M1|M5|M15|M30|...
 - **ModelType**: model, 0=EveryTick|1=ControlPoints|2=BarOpen
 - **ModelQuality** : 99.9
 
-
 ``` Golang
 // FXTHeader History Files in FXT Format
 // https://www.metatrader4.com/en/trading-platform/help/autotrading/tester/tester_fxt
@@ -209,7 +209,7 @@ start := time.Date(2017, time.January, 10, 22, 0, 0, 0, location)
 end := start.Add(4 * 24 * time.Hour)
 
 // Create new stream that each tick will only be called within start and end boundary
-stream := stream.New("GBPJPY", start, end, createEmptyDir(t)) 
+stream := stream.New(instrument.GetMetadata("GBPJPY"), start, end, createEmptyDir(t)) 
 
 // Stream ticks with the requested parameter
 stream.EachTick(func(time time.Time, tick *tickdata.TickData, err error) bool {
@@ -231,7 +231,7 @@ start := time.Date(2017, time.January, 10, 22, 0, 0, 0, location)
 end := start.Add(4 * 24 * time.Hour)
 
 // Create new ticks that iterate within start and end boundary
-ticks := New("GBPJPY", start, end, createEmptyDir(t))
+ticks := New(instrument.GetMetadata("GBPJPY"), start, end, createEmptyDir(t))
 
 for !ticks.IsCompleted() {
     isSuccess, err := ticks.Next() // Ticks start before start, must start with Next(). 
@@ -243,4 +243,24 @@ for !ticks.IsCompleted() {
     isGotoSucceed, err := ticks.Goto(tick.UTC().Add(time.Hour)) // Skip one hour forward
     tick = ticks.Curr()                                         // tick is now the first tick after an hour later
 }
+```
+
+## 6 Downloader API (From v0.3)
+
+Prepare cache folder before running ticks or stream API.
+
+``` Golang
+d := NewTickDownloader(folder)
+
+from := time.Date(2021, time.January, 8, 0, 0, 0, 0, time.UTC)
+to := from.Add(time.Hour)
+d.Add(instrument.GetMetadata("EURUSD"), from, to).
+	Add(instrument.GetMetadata("GBPUSD"), from, to)
+
+d.Download(func(instrumentCode string, dayHour time.Time, err error, curr, count int) {
+    // dayHour is the bi5 download
+    // err is the download error. nil if there's none
+    // curr is the current download count
+    // total is the total number of bi5 to download
+})
 ```