Add github extension and bug fixes

- add github extension which includes extra and all extensions that
provide a github feel
- b64 should return full tag
- headeranchor should use unique ids
- toc should always occur before headeranchor (usability)
Keep extra separate
One command to rule the all

- Add MarkdownPreviewSelectCommand
- Quick panel option is now only MarkdownPreviewSelectCommand

When multiple parsers are enabled in “enabled_parsers” the user will be
prompted for which parser to use.  If only one is defined in the list,
that parser will automatically be run.
Merge pull request #212 from facelessuser/master

Bug Fixes and Usability Changes