Fix for embedding code from github
Add option for custom js (like custom css)

* Add a new 'js' flag to indicate you want to embed a file named 'custom.js'
  which should exist at the root of the wiki
Merge pull request #661 from simonista/custom-js

Add option for custom js (like custom css)
Update README.md
Update README.md
Merge pull request #660 from simonista/fix-github-links

Fix for embedding code from github
Merge remote-tracking branch 'gollum/master'

Conflicts:
	bin/gollum
	lib/gollum-lib/markup.rb
	lib/gollum/frontend/app.rb
	lib/gollum/frontend/templates/file_view.mustache
	lib/gollum/frontend/templates/layout.mustache
	lib/gollum/frontend/views/layout.rb
	test/test_app.rb