Style only

Conflicts:
	yi/src/library/Yi/Buffer/Misc.hs
Prompt the user for a comment string when missing.

Just like in emacs, we let the user to pick a string to use for comments
if the mode doesn't already define one. We remember this by modifying
the current mode for the buffer on the fly with the comment syntax the
user picked. This change is buffer-local and doesn't affect any other
buffers: we don't want to force the comment symbol in every other buffer
as well!

Unfortunately due to suddenly needing the YiM type and few buffer
functions in scope in Yi.Buffer.Misc, there are quite a lot of boot file
changes but it's either this or having to create a new module in a
rather odd place.

Fixes #413.

Conflicts:
	yi/src/library/Yi/Keymap/Emacs.hs