Add background variants for term-color-*

[bradwright]

These have defaults which don't look right in ansi-term etc.

They're also safe to overwrite the background color of because each variant applies to the ANSI equivalent of background/foreground, e.g text won't suddenly become entirely orange.

Without this change you get yellow background like the one seen on master:

Prior to this change running M-x describe-face term-color-yellow shows the following:

Face: term-color-yellow (sample) (customize this face)
...
Foreground: #a57705
Background: yellow3