Improved keybinding / command API

[aidanholm]

API should allow:

1. Completion of arguments, based on argument type. E.g. :open <uri>, :buf <tab_uri>, :forward/:back <tabhist_uri>; See #76. Probably implemented through a format string.
2. Partial keybind completion. Should support showing an error if the input buffer doesn't prefix any valid keybinding, instead of filling the input buffer. See #111
3. Decoupling action and keybinding? E.g. there are multiple scrolling binds with duplicated function and description differing only in the bound key; it would be nice to fix that.
4. Better way of overriding module-provided commands / keybinds. E.g. noscript.lua provides ,ts, ,tp, and ,tr, which currently are difficult to remap. May tie in to decoupling actions and binds.