Add test for Use per-post permalinks ending in .htm
Allow per post permalinks to end in `.htm`
Fixes #2920
Fix code example for media query mixin

It was using an old variable name (`$palm`) instead of the current one (`$on-paln`).
Rouge's Redcarpet plugin now has a different API

See for more:
- https://github.com/jneen/rouge/commit/18ec938e9ca9463fb424c3d706f8fc761dad2460
- https://github.com/jneen/rouge/issues/182
Latest version of Rouge so we know the API is right.
Merge pull request #2951 from jekyll/fix-rouge
Update history to reflect merge of #2951 [ci skip]
Merge pull request #2946 from kleinfreund/patch-1
Update history to reflect merge of #2946 [ci skip]
Merge pull request #2925 from pathawks/htm-extension
Update history to reflect merge of #2925 [ci skip]
Make permalink parsing consistent with pages

Require permalink to end in a slash to write an index file.
Addresses #2920.
Update Cucumber tests to match new permalink parsing
Merge pull request #2921 from alfredxing/post-dest
Update history to reflect merge of #2921 [ci skip]