Don't process quickfix-loaded files, closes #176

Certain quickfix-commands like vimgrep have to load all searched files
into Vim and will execute the BufReadPost autocmd for all of them. Since
Tagbar doesn't need to generate tags for those files pause processing
while the command is running.

There doesn't seem to be a better way to find out whether vimgrep is
running than to set a temporary variable with the QuickFixCmdPre/Post
autocmds, see also
https://groups.google.com/forum/#!topic/vim_use/sUj5MFGmwD8