Display available tutorials (rstudio#234)

* add an available tutorials method

* return unique tutorials only

* Tests for available_tutorials

* check if package exists

* make available tutorials invisible when returning from run_tutorial

* add `stop.` which equals `stop(..., call. = FALSE)`

* add tests

* document

* add close matching if within 3 letters

```
❯❯ dev_load(); run_tutorial("helloo", "learnr")
Loading learnr
Error: 	Tutorial "helloo" was not found in the "learnr" package.
	Did you mean "hello"?
	Available "learnr" tutorials: "hello", "question_type", "slidy"
```

* import adist from utils

* added new news item and rearranged the news order

* ignore case within adist to give best match

* give slidy a title

* comment why using 'missing'

* add an example in run_tutorial.  Also display all available learnr tutorials

Fixes rstudio#223

* use NULL default values for run_tutorial to avoid "missing" issues

* extract out available_tutorial methods to a single file. Add formatting methods

return an invisible data.frame of the information

* cleaner error formatting

* minor code feedback

slidy/slidy.Rmd

@@ -1,6 +1,7 @@
 ---
 output: slidy_presentation
 runtime: shiny_prerendered
+title: "Slidly demo"
 ---
 
 ```{r setup, include=FALSE}
@@ -13,7 +14,6 @@ library(dygraphs)
 Modify the dyOptions to customize the graph's appearance:
 
 ```{r dygraph-options, exercise=TRUE, exercise.eval=TRUE, fig.height=5.5}
-dygraph(ldeaths) %>% 
+dygraph(ldeaths) %>%
   dyOptions(fillGraph = TRUE, drawGrid = TRUE)
 ```
-