More contrast for Cursor and MatchParen

Cursor is dark text on a light background
so that the character underneath can be read.

MatchParen is light on dark to distinguish it
from the cursor.

Fixes #30. Also a reversal on #11.
Adjust highlight colors for black background

To make 'search' and 'colorcolumn' highlighting visible when using a
black backround the corresponding highlight colors are adjusted.

Signed-off-by: Sören Brinkmann <soeren.brinkmann@gmail.com>