Functionality implemented:
- :tgroupcreate to create a tab group
- :tgroupswitch to switch to or create a tab group
- :tgrouplast to switch to the previously active tab group
- :tgrouprename to rename the current tab group
- :tgroupclose to close all tabs in the current tab group
- :tgroupmove to move the current tab to another tab group
- :tgroupabort to destroy all tab group information and show all tabs
- :tabgroup... aliases for the above commands
- Completion for :tgroupswitch and :tgroupmove
- Restoring tab group setup on firefox start (seems to just work without any extra code)
- Basic mode indicator
- Setting for initial urls for to use newly created tab groups (after first)
- Setting for whether :tab shows hidden tabs.
Edge cases handled
- Opening tabs outside of tridactyl mechanisms (e.g. firefox gui, C-t, etc.)
- Switching to a hidden tab
- Detaching and attaching tabs
- Pinned tabs (they can't be hidden; the commands ignore them)
- Various other edge cases
Fixes#811.
* Consistent semicolons at end of solid bullet points under Permissions header.
* Bullets under Highlighted Features start with a capital letter and end with a period.
* Changed first line to bold because ## header syntax is not working.
