feat(lib): exporting metrics by default

This allows to require metrics easier than before by doing:
```
const metrics = require('pwmetrics/lib/metrics');

// {
//     TTFCP: "firstContentfulPaint",
//     TTFMP: "firstMeaningfulPaint",
//     TTFCPUIDLE: "firstCPUIdle",
//     TTI: "interactive",
//     SI: "speedIndex"
// }
```

Contrary to the current way:
```
const metrics = require('pwmetrics/lib/metrics/metrics');

// {
//     METRICS: {
//         TTFCP: "firstContentfulPaint",
//         TTFMP: "firstMeaningfulPaint",
//         TTFCPUIDLE: "firstCPUIdle",
//         TTI: "interactive",
//         SI: "speedIndex"
//     }
// }
```

The `readme.md` and `gulpfile.js` files have been updated too.

lib/metrics/index.ts

@@ -0,0 +1,3 @@
+import { METRICS } from './metrics';
+
+module.exports = METRICS;

readme.md

@@ -110,7 +110,7 @@ module.exports = {
 `pwmetrics-config.js`
 
 ```js
-const METRICS = require('pwmetrics/lib/metrics/metrics');
+const METRICS = require('pwmetrics/lib/metrics');
 
 module.exports = {
   url: 'http://example.com/',

recipes/gulp/gulpfile.js

@@ -5,7 +5,7 @@
 /* eslint-disable no-console */
 const gulp = require('gulp');
 const PWMetrics = require('../../lib/');
-const {METRICS} = require('../../lib/metrics/metrics');
+const METRICS = require('../../lib/metrics');
 
 /**
  * Run pwmetrics