Replace s:active with b:vimroom_enabled

- Using a buffer local variable allows each individual buffer to manage its own
  VimRoom state
Move GetHighlighting closer to SetVimRoomBackgrnd
Change b:vimroom_enabled to t:vimroom_enabled

- Change scope of enabled variable from buffer to tab.  This allows the opening
  of multiple buffers in the VimRoom environment
- Instead of VimRoom being a transformation on a buffer, it's more like opening
  an auxiliary program in Vim and giving that program a dedicated tab
Change saved local options to buffer-scoped vars

- Goes with previous change, if VimRoom can be applied to multiple buffers, then
  each buffer needs to be able to keep track of its own local variables
Extract VimroomToggle logic into 2 separate funcs

- Create SetupVimRoom and TeardownVimRoom, these functions will assist the
  autocommands in preserving the state of VimRoom when changing buffers, tabs,
  or colorschemes.
Add autocmds to preserve VimRoom state
Merge branch 'autocommands'