The default colorschemes should ideally use 256 colors instead
of just 16 colors. The `simple` colorscheme should cover most
16 color use cases. I went through the colorschemes and put the themes
that didn't look good or looked incomplete in an in_progress directory.
This commit also improves the `cd` command behavior when using an
unnamed buffer.
Use the 'regexp2' library for lookahead and lookbehind in region
start and end regular expressions to support things like closing quotes
that aren't preceded by backslashes.
