Rewrite to use babel

Previously, regexes were used to inspect and manipulate the example.
This commit rewrites the whole plugin to instead properly depend on
ASTs (yay!)  This also comes with the benefit of asynchronous file
system operations.  And lastly, many new tests, support for multiple
console results, better temp file management, and much better
internals.

We no longer support `options.cwd`.  Instead, set the `cwd` on the
processed file to that instead.

.prettierignore

@@ -0,0 +1,3 @@
+coverage/
+example.js
+test/fixtures/fail-could-not-parse-package/index.js

.remarkignore

@@ -0,0 +1,2 @@
+example/
+test/

example.js

@@ -1,26 +1,36 @@
-// This section is rendered by this module from [example.js][example-js].
+// > This section is rendered by this module from [`example.js`][example-js].
+// > Turtles all the way down.  🐢🐢🐢
 
-// Dependencies:
-var fs = require('fs')
-var remark = require('remark')
-var usage = require('./index.js') // This is changed from `./index.js` to `remark-usage`
+// remark-usage-ignore-next 4 - Get the files to show them in our usage example.
+var readFile = require('fs').readFileSync
+var join = require('path').join
+var exampleJs = readFile(join('example', 'example.js'), 'utf8')
+var exampleMd = readFile(join('example', 'readme.md'), 'utf8')
 
-// Read and parse `readme.md`:
-var readme = fs.readFileSync('readme.md', 'utf-8')
-var ast = remark()
-  .use(usage)
-  .parse(readme)
+// remark-usage-ignore-next - Use async/await so it looks nicer.
+;(async function (){
 
-// Log something with a language flag:
-console.log('markdown', remark().stringify(ast.children[2]))
+// Say we are making a module that exports just enough Pi (3.14159).
+// We’re documenting it with a readme file, [`example/readme.md`][example-md]:
+console.log('markdown', exampleMd)
 
-// Or without language:
-console.log(remark().stringify(ast.children[3]))
+// …and an example script to document it [`example/example.js`][example-js-2]:
+console.log('js', exampleJs)
+
+// …If we use `remark-usage`, we can generate the `Usage` section
+var path = require('path')
+var vfile = require('to-vfile')
+var remark = require('remark')
+var usage = require('.')
+
+var file = vfile.readSync({path: 'readme.md', cwd: 'example'})
+
+var file = await remark()
+  .use(usage)
+  .process(file)
 
-// Log something which is never captured:
-function neverCalled() {
-  console.log('javascript', 'alert("test")')
-}
+// Now, printing `file` (the newly generated readme) yields:
+console.log('markdown', String(file))
 
-// Log something which isn’t captured because it’s not a string.
-console.log(this)
+// remark-usage-ignore-next
+}())

example/example.js

@@ -0,0 +1,5 @@
+// Load dependencies:
+var pi = require('.')
+
+// Logging `pi` yields:
+console.log('txt', pi)

example/index.js

@@ -0,0 +1,3 @@
+'use strict'
+
+module.exports = Math.floor(Math.PI * 1e5) / 1e5

example/package.json

@@ -0,0 +1,5 @@
+{
+  "private": true,
+  "name": "pi",
+  "main": "./index.js"
+}

example/readme.md

@@ -0,0 +1,9 @@
+# PI
+
+More than enough 🍰
+
+## Usage
+
+## License
+
+MIT