Implemented extension customization from options - closes #20

Below is a suggestion for updating the documentation at http://angular-gettext.rocketeer.be/dev-guide/extract/

# Using custom file extensions

If the views to translate are in files with a specific extension (for instance `*.ctp`), you can override the default extensions mapping by updating the `nggettext_extract` options accordingly:

```js
grunt.initConfig({
  nggettext_extract: {
    pot: {
      options: {
        extensions: {
          html: 'html',
          ctp: 'html',
          js: 'js'
        }
      },
      files: {
        'po/template.pot': ['src/**/*.js', 'src/views/*.html', 'src/views/*.ctp']
      }
    },
  },
})
```

The `extensions` option must be an object whose keys is the extension and value is a valid extraction strategy (so far either `'html'` or `'js'`).
It will override default values, so be sure to add all extensions to parse.