Added partial support for multimarkdown definiton lists

Definiton lists support is strictly a one-line per definiton, which
isn't totally to spec but good enough for my purposes.
Enabled 'github' and my definition list extensions

Enabled 'github' and 'definiton' showdown extensions by default.
Added definition list CSS for mdoc

• Tweaks for my preferences (e.g. code background colors)
• Added two definition list styles:
1) line definiton lists (lists that start with code backticks)
2) regular lists