Add (experimental) ASCII graph-drawing mode.

This works in one of two ways:

- run a command, the output of which produces floating point values, one
  per line; press "g" to graph (only) these values on each run

- run a command with a pattern that selects a floating point value;
  press "g" to graph this single value as it changes over time

This definitely needs some more work, but it's at least demoable in its
current state.