Add cterm, term', and gui` to g:jellybeans_override

[docwhat]

Add the missing highlight attributes cterm, term, and gui to g:jellybeans_override.

You could also modify the override to look at attributes and set whatever the attributes that exist, looking for a 256 prefixed version for things that begin with `c.

This would work best when you have done #32 and #37.

This code may be helpful: How to modify existing highlight group in vim?

[nanotech]

g:jellybeans_overrides will already set both gui and cterm from an attr key, as in the readme example. Is that enough to suit your use case?

[docwhat]

Unless I read the code wrong it only works for the fg and bg versions. Can you set italic or bold with those?

[nanotech]

Yep. Before:

After:

[docwhat]

attr? Sigh, can't anything with terminals be simpler?