Ideas:

How to depict stereo wedges and dashes?

Maybe we could change the first bond character, or maybe the first half of the
bond, to a ^ or v instead of a dot, to signal if the bond goes up or down
relative to the beginning atom.

We could also use bold, reverse, or color. But in any case a tricky part is
that the bond is directional; we can't simply tag the entire bond.

What other characters might we use?

From pure ASCII:

    : second half of dotted bond?
    | second half of solid wedge?
    ,
    >
    <
    "
    '
    `
    * (better for dummy atoms)
    ~ (better for wiggly bonds)

    Cvvv...C
    C^^^...C

From Unicode maybe arrows?

    ↑
    ↓

or bullets? Let's go with that (solid bullet / empty bullet / middle dot)

TODO:

- Add a "fit to screen" command?
- Improvements to bond selection: make the angle tolerance a function of
  distance.
- There's a stereochemistry bug! Cleanup can cause epimerization! We need to
  assign stereo before doing cleanup.
- Capture Ctrl-C?
