Mention the 'input' option to use a GFM parser in the kramdown options documentation.
move input GFM option down to the bottom, not in the defaults
Update files list in gemspec.

Fixes #1793
Merge pull request #1791 from konklone/kramdown_gfm

Document Kramdown's GFM parser option
Update history to reflect merge of #1791
Fix a broken link on the troubleshooting.md page
move EntryFilter class from global namespace into Jekyll module
Merge pull request #1797 from dougjohnston/patch-1
Update history to reflect merge of #1797
Merge pull request #1800 from cs/move-entry-filter-into-jekyll-module
Update history to reflect merge of #1800