Support comparators for matching keyboard input to meta and ctrl keys

[jquast]

The currently proposed keystroke_input implementation in the blessed-integration branch does not provide any mechanism to verbally describe Ctrl-c, or Meta-c.

This issue derived from the following TODO items (removed):

        # TODO(jquast): "meta sends escape", where alt+1 would send '\x1b1',
        #               what do we do with that? Surely, something useful.
        #               comparator to term.KEY_meta('x') ?
        # TODO(jquast): Ctrl characters, KEY_CTRL_[A-Z], and the rest;
        #               KEY_CTRL_\, KEY_CTRL_{, etc. are not legitimate
        #               attributes. comparator to term.KEY_ctrl('z') ?

as well as #27 (comment)
and #44