Add support for scanDirs globbing

It is now possible to say

```js
    lingua: {
      extract: {
        potDest: 'translations/messages.pot', // dest path must exist
        scanDirs: [
             // everything recursively under app/
             '<%= yeoman.app %>/**'
             // but not 3rd party components
           , '!<%= yeoman.app %>/bower_components/**'
        ]