refactor file+matter creation in cucumber steps
refactor step to support page creation
add failing test for sorting pages by custom variables
clean up custom variable sort feature
allow data attribute access by Liquid on Convertible items
make non-data properties/methods accessible to Liquid

per https://github.com/jekyll/jekyll/pull/1849/files#r8490593
Merge remote-tracking branch 'upstream/master' into sort-attributes
fix file sorting test
Merge pull request #1849 from afeld/sort-attributes