Add support for searching using regex

For commits the title, author, and displayed date string is searchable.
Search text is not highlighted, the current line is simply used to jump
around and show which line matched.

Default keybindings:

	'/'	search forward
	'?'	search backward
	'n'	find next
	'N'	find previous

Note, this means that '?' no longer displays the help view, just use 'h'.