Allow creation of moment+langs.js

Represents a continuation of #973 (didn't mean to create a new pull request specifically, but all the old work was made irrelevant anyway, and apparently github auto-closes pull requests if you delete the branch.  Go figure).

moment+langs.js is a version of moment.js, prepackaged with all languages.  Can specify --embed_languages=ar,br to create min/moment+customlangs.js with just those two languages (went with this over min/moment+ar+br.js since it was easier to configure and minify; could switch over if that's still your wish).  Also didn't indent the lang closures, since I think the file is readable as is; again, could indent if you guys want.  I think that about covers everything.