lookup_db.py: Add --colour option to force coloured diff output

A typical example from a Unix environment (such as cygwin or Linux):

lookup_db.py *.txt --color | less -R

--color is accepted as well as --colour
lookup_db.py: Add --no-diff option to suppress diff output

This can be used to show the raw shader for copy+paste.

Going forward I'd like to add the ability to save all matched shaders to
files, then throw an interface on this so the user can pick and choose.