python/colour: add colourizing and switch functions

python/colour: add colourizing and switch functions

When samba.colour is first imported, the function

colour.c_BLUE("samba")

will give you the string "\033[1;34msamba\033[0m", which will show up
as blue on an ANSI terminal. If you then go:

colour.switch_colour_off()
colour.c_BLUE("samba")

the c_BLUE call will return the uncoloured string "samba".

This is so things like samba-tool can do this sort of thing:

    if not os.isatty(self.outf):
        switch_colour_off()

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>