Add ansi theme to replace ansi-light and ansi-dark

This combines ansi-light and ansi-dark into a single theme that works
with both light and dark backgrounds. Instead of specifying white/black,
the ansi theme uses the terminal's default foreground/background color
by setting alpha=01, i.e. #00000001. This is in addition to the alpha=00
encoding where red contains an ANSI color palette number.

Now, `--theme ansi-light` and `--theme ansi-dark` will print a
deprecation notice and use ansi instead (unless the user has a custom
theme named ansi-light or ansi-dark, which would take precedence).

CHANGELOG.md

@@ -18,6 +18,8 @@
 
 ## New themes
 
+- `ansi` replaces `ansi-dark` and `ansi-light`, see #1104 and #1412 (@mk12)
+
 ## `bat` as a library
 
 ## Packaging