Adds the collection label to a document's data

Related to issue [#52](https://github.com/jekyll/jekyll-help/issues/52) in Jekyll/Jekyll-help this PR adds a document's collection label to its available data.

Forgive the noob, please: I tried adding a test test_document.rb in 'should know its data' and it failed, but I think because of the test, not the change. I ran the addition on a local install and it worked there.