Add option to hide previous/next modes in ctrlp

I don't like seeing the previous and next mode indicators in ctrlp. I find they add noise and make it hard to tell what the current mode and settings are (especially since they're the same colour as settings). Without airline, I can change the function that creates the status bar, but I can't see how to fix this without modifying airline. So I added an option.

New option airline#extensions#ctrlp#show_adjacent_modes allows users to
toggle showing the previous and next modes. The default is the same
behavior as before: show the modes.

Add documentation for new option.

These modes are useful if you switch forward and back through ctrlp's
functionality, but they are visual noise if you don't.