implement keyboard-awareness

this completes the proposed API to provide blessings with keyboard
input. this performs the expected heavy lifting of translating special
multibyte sequences representing application keys to a unicode-derived
instance of Keystroke().

awaiting feedback from Erik Rose.

Yet to remain:
  - test cases: this may require bringing in the 3rd party dependency of
    'pexpect'; or, by rolling a more advanced version of @as_subprocess
    decorator.
  - example code within README.rst
  - bugs ?!?