Add support for scanDirs globbing

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