Use shell-mode CWD in uniquify-buffer-name-style

[Chris Warburton]

Emacs recently allows uniquify-buffer-name-style to be a function, which can
return a unique buffer name in the desired format. For example, instead of
'*shell*<3>' we can use this function to spot shell-mode, get the CWD and return
'my-dir', 'my-dir<2>', etc. instead. Saves a lot of manual renaming.

Also, can be called at any time via rename-uniquely (C-x x u), e.g. once we've
cd'd to a different directory!