Quick indent lists

[alehandrof]

I had to cherry pick through my devel branch. I hope this is everything :)

• Resolves #9.
• The regex looks for most of what Pandoc would consider valid. (See https://github.com/alehandrof/SmartMarkdown/commit/90953bf8eac5b4d6590f249e0af775c398783a0e for details.)
• @vovkkk had suggested I restrict the binding so that it doesn't fire inside code blocks. I haven't found this necessary, but if you decide otherwise, this is an easy fix.
• I have added the ability to use Ctrl+Enter to add new line with SmartList when in a list. I find this very useful, but YMMV. (https://github.com/alehandrof/SmartMarkdown/commit/b79c9ec35d50186fceca5a2b427f0b88148dcd9f)