Split income from Short Term Capital Gains

[hickeng]

It turns out that at least one state has handling of STG distinct from income, so it'd be useful for some subset of people to have this split. For the vast majority of people, the only income that'll be incurred is if the fractional share comes from an ESPP lot and that's a tiny impact. That makes this comparatively low priority, but should still do it for correctness.

The maximum rate for short-term capital gains in the state of Massachusetts is 12.5%