Extend to search and parse multiple message directories

[euwest]

allows for passing localeDirs(an array of paths to directories containing locale directories with po files in them) to create message dictionaries by merging same-language pos.

example:

var options = {
    localeDirs: [
        'locale', 
        'node_modules/some-module/locale',
        'bower_components/some-component/locale'
    ]
};

gulp.task('translate', function(){
  return gulp.src('collected_static')
    .pipe(statici18n(options));
});

the first directory in localeDirs is treated as the canonical source, which means the languages the app is translated into are derived from the contents of this directory, and the messages from .po files in it are extended but not overwritten by proceeding directories.

[euwest]

@beck

[euwest]

:bell:

[beck]

:+1: