Mention extensions option on documentation

[rubenv]

[joh-klein]

To get you started :) (and for others looking for some help):

'nggettext_extract': {
    pot: {
        options: {
            startDelim: '{{',
            endDelim: '}}',
            lineNumbers: true,
            postProcess: function(po) {}
        },
        files: {
            'strings.pot': [*.html]
        }
    }
}

The most fun is with the postProcess option possible …