Add test for Use per-post permalinks ending in .htm
Allow per post permalinks to end in `.htm`
Fixes #2920
Add note about using the github-pages gem properly
Move it down a little
Fix code example for media query mixin

It was using an old variable name (`$palm`) instead of the current one (`$on-paln`).
Make it a little nicer.
Merge branch 'ghp-gem' of github.com:jekyll/jekyll into ghp-gem

* 'ghp-gem' of github.com:jekyll/jekyll:
  Move it down a little
Merge pull request #2939 from jekyll/ghp-gem
Update history to reflect merge of #2939 [ci skip]
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]
Merge remote-tracking branch 'upstream/master' into patch-9