Add "secondary" color option

[an-prata]

Adds a "secondary" color option similar to "auto" to specify the second highest weighted color of the album artwork. "secondary" is implemented in largely the same way as "auto".

Apologies in advance if this feature was omitted intentionally.

[GuardKenzie]

Thanks for this pull request; everything looks good! I'll take a better look and hopefully merge this next week/weekend. 🧡

Since this overwrites another curses colour pair, I'll also add a note in the README that makes users of terminals that don't reset properly aware of that (see #34).

[an-prata]

Have you decided not to merge this because of the plans for v2? Asking since its been a while. I know v2 would eventually just overwrite this pr but for the meantime it may be nice to have the feature. I've been using my fork on my desktop for a while and it would be nice to get back to using the AUR package.

[GuardKenzie]

Sorry for the late reply! I was hesitant to merge this because it would exasperate #34, however, since I'm working on v2 which will (hopefully) resolve that I'll go ahead and merge.