lib/color: add colorscheme parser
hexdump: add highlighting support

[kzak@redhat.com: - fix coding style,
                  - use xalloc in all code,
		  - fix strtol usage]

Signed-off-by: Ondrej Oprala <ooprala@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>