Merge the two color schemes into one scheme with a light and dark mode

For your consideration: these commits merge the seoul256 and seoul256-light schemes into a single seoul256 scheme. The two versions can be switched between via 'set background=dark' and 'set background=light'. I believe I've preserved the behavior of both themes.

Basically, `s:hi` now takes lists for colors, where the first item in the list is the dark color, and the second is the light. It then uses `&background` to decide which of those to use. The other major change is that I changed the various `s:seoul256_background` variables into `s:dark_bg` and `s:light_bg` versions, so that both can be referenced in the subsequent code.

One thing I didn't do was split the g:seoul256_background into two pieces. It might make sense to have one setting for each version of the scheme.

If you want to keep the two files split, that's fine, too. This is just how I wanted it to behave for my own use, and I thought I'd give the opportunity to merge back, since the work has already been done.

Cheers.