Add functionality for disabling and enabling Ycm autocompletion.

This is the better solution for this pull request that i made this morning https://github.com/Valloric/YouCompleteMe/pull/1165

On that PR autocmd was triggering even when Lock command was called. In this PR that is fixed, so now when you call `YcmLock` it disables autocommands for CursorMoved and CursorMovedI.

This also should help for issue https://github.com/Valloric/YouCompleteMe/issues/1164
"Lock" and "unlock" are fairly meaningless. Also, these aren't targeting direct user usage so don't make them into commands. Just make them public functions (so use `youcompleteme#` function prefix).

Name these `youcompleteme#DisableCursorMovedAutocommands` and same but `Enable`.