Make colour output when printing Params objects optional

[lazappi]

Mostly this is an issue for running devtools::test() which captures the colour codes and means regexes in expect_output() don't match (devtools::check() doesn't do this for some reason).

Could be done by providing an argument to show methods or some environment option. Another option might be to invisibly return a string which can be passed to crayon::strip_style() but I think that has other issues.