Add flag to toggle detection at runtime

[bmundt6]

This makes it possible to enable or disable detection during an autocmd event, which is useful as detection can be slow on extremely large files.

[bmundt6]

For clarity, this is specifically how I'm using the toggle in my .vimrc:

let g:conflict_marker_enable_detect = 0
autocmd User VimConflicted let g:conflict_marker_enable_detect = 1 | doautocmd FocusGained

Now, conflict marker detection is only used when the Vim Conflicted plugin is active.

[rhysd]

Thanks. I merged this branch after small follow up at d5937a7.