Add support for setting color options in the ~/.tig user configuration file

Add support for setting color options in the ~/.tig user configuration file

Now you can put stuff like this in ~/.tig:

	# Diff colors
	color diff-header	yellow	default
	color diff-index	blue	default
	color diff-chunk	magenta	default
	# UI colors
	color title-blur	white	blue
	color title-focus	white	blue	bold