Bug Fixes and Usability Changes

Bug Fixes:
- b64 should return full tag when conversion not performed
- headeranchor should use unique ids
- toc should always occur **before** headeranchor

Usability Changes:
- github extension added, which is a shortcut for including extensions to add to the github feel of the render: `headeranchor`, `tasklist`, `nl2br`, `delete`, `githubemoji`, and `magiclink`
- quickpanel presents one command opposed to github and python markdown (uses `MarkdownPreviewSelectCommand`)
    - `enabled_extensions` allows the user to configure which parsers they can pick out of
    - When multiple parsers are available, the user will be prompted via the quickpanel which parser to use
    - If only one parser is in the list, the the command will automatically be run.