adding a proper plugin for i18n and making it work with yfm templates

This PR creates an actual plugin to generate the pages collection after the configuration has occurred so we don't have to worry about grunt processing the yfm too soon.

Some things still need to be worked on here (like making assemble work even if no source and no pages are supplied at first). These things though should be resolved in the next version of assemble and this plugin can be updated then.