Allow sorting by custom properties

Fixes https://github.com/jekyll/jekyll/issues/1802.

Added a failing test to support `sort:'some-custom-property'`.  Have spent a couple hours poking around Jekyll/Liquid to figure out how to make those properties available to the filter, but haven't been able to figure it out yet.

Presumably a fix would work for other filters as well?  Not sure how to test more generically.

P.S. These things are called "variables" in http://jekyllrb.com/docs/frontmatter/, but are called "properties" in the Liquid docs (e.g. in [`sort`](http://docs.shopify.com/themes/liquid-basics/output#sort)).  Sorry for my inconsistency... I'll clean up later.

/cc @parkr @Haacked