New buffer explorer mode with highlighting and more

Integration of [ctrlp-hibuff](https://github.com/sergey-vlasov/ctrlp-hibuff) as [requested](https://github.com/sergey-vlasov/ctrlp-hibuff/issues/1).

The new buffer explorer replaces the original functionality and adds new things as:
* Highlighting of everything (buffer name for each buffer state, buffer numbers etc.)
* Showing buffer numbers and paths
* Quick match by modified buffers using ```+``` indicator, matching by buffer number or name
* Single/multiple buffer deletion (like in MRU mode via F7)