[Feature] Add more color options.

[carnager]

Having colors in ncmpc is good, but right now a bit limited.
I suggest having more color types, like:

basic ones:
color title-active
color-title-bold-active
color separator (for the separator used in columns mode, which right now uses the value of the line color)
color-list-active

and maybe even some more advanced for the main area:
color-tag-TAGNAME

[MaxKellermann]

What is an "active" title/list?