support archiving by `lang` attribute

[dreamalligator]

would be nice feature to specify a front-matter key to archive by.

example:

tags: [something, else, entirely]
category: Monty Python
language: French

and this post would be archived on the "French" page

[dreamalligator]

I've updated the title to reflect that jekyll-feed supports the lang attribute already. https://github.com/jekyll/jekyll-feed/blob/master/lib/jekyll-feed/feed.xml#L36