Experimental option `g:diminactive_use_syntax`

Ref: https://github.com/blueyed/vim-diminactive/issues/6
(Re)store original syntax
Add g:diminactive_use_colorcolumn

This allows to use e.g. `g:diminactive_use_syntax` alone.
Keep syntax on for buffers visible in multiple windows
DimInactiveCallback: ignore non-normal buffers (&buftype!="")
Setup: do not process active tab twice
Fix/improve init (honor SessionLoad), add b:diminactive with commands

New commands: DimInactiveBufferOff / DimInactiveBufferOn to toggle state
of the current buffer.
Merge branch 'use-syntax'

Fixes https://github.com/blueyed/vim-diminactive/issues/6.