McFly - fly through your shell history

McFly replaces your default `ctrl-r` shell history search with an
intelligent search engine that takes into account your working directory
and the context of recently executed commands. McFly's suggestions
are prioritized in real time with a small neural network.

TL;DR: an upgraded `ctrl-r` where history results make sense for what
you're working on right now.

Features:

  - Rebinds `ctrl-r` to bring up a full-screen reverse history search
  prioritized using a small neural network.

  - Augments your shell history to track command exit status, timestamp, and
  execution directory in a SQLite database.
