Add Option to exclude nonmodifiable buffers from MRU

For example, when searching for `.txt` the MRU is crammed by recent accesses to the VIM doc by `K`. With this addtitional option `g:ctrlp_mruf_exclude_nomod`, these files, as well as all other nonmodifiable buffers are no longer added to the MRU list.

I also cleaned up the autocmd events for the MRU list. Please check that it suits you. Regardless, the BufWinEnter autocmd is crucial to check at the right time if the buffer is nonmodifiable.