Messages source code parser

We should parse code with [esniff](https://github.com/medikoo/esniff#esniff).

I think it may make sense to create `parse` directory, and create following scripts:

1.  `parse/source`, it'll assume it'll receive content of one file, and return following map:
```javascript
{
  context: 'fooContext',
  messages: {
    "Normal message": [{ point: 17, column: 17, line: 2 }, { point: 37, column: 17, line: 4 }],
    "n\0One Apple\0Many apples":  [{ point: 17, column: 17, line: 2 }]
  }
}
```
2. `parse/paths`, it should work similarly to how xlint behaves, but with no fancy things like streaming and watching. Technically behind a scenes, on each path it receives, it should run [fs2/readdir](https://github.com/medikoo/fs2/#readdirpath-options-cb-fs2readdir) with following (overridable) options:
`{ depth: Infinity, type: { file: true }, pattern: /\.js$/, ignoreRules: 'git' }`
Behind the scenes it should rely on `parse/source`

It should return following map:
```javascript
{
  "Normal message":  {
    fooContext:  [{ fileName: '/path/to/filename/foo.js', point: 17, column: 17, line: 2 },
      { filename: '/path/to/filename/foo.js', point: 37, column: 17, line: 4 }],
    default:  [{ fileName: '/path/to/filename/bar.js', point: 17, column: 17, line: 2 }]
  }
  "n\0One Apple\0Many apples":  {
    fooContext:  [{ fileName: '/path/to/filename/foo.js', point: 17, column: 17, line: 2 },
      { filename: '/path/to/filename/foo.js', point: 37, column: 17, line: 4 }],
    default:  [{ fileName: '/path/to/filename/bar.js', point: 17, column: 17, line: 2 }]
  }
}
```

Let's add an option where it would be decided what prefix (`i18n`) should be used,  and default it to what's now