Add support for keybindings

The syntax is

	bind <keymap> <key> <request>

Keybindings are resolved by first looking in the keymap of the active view,
then in the generic keymap, and finally in the built-in default keybindings.

Examples to put in ~/.tigrc:

	bind main w scroll-line-up
	bind main s scroll-line-down
	bind diff a previous
	bind diff d next
	bind diff b move-first-line
	bind generic g view-next