feat: configurable colours

[justdan96]

I have been using dyff for a while and really like it but the default colours are not particularly friendly for people with a colour vision deficiency (such as myself). It would be nice if this could be configured somehow, even if it is just through some environment variables that I can define. To give you an idea of what I mean for delta I can configure the style like so:

# Fabio Crameri, Glasgow palette, 5 colours, 2 for plus and 4 for minus
minus-style                   = syntax "#702d06"
minus-non-emph-style          = syntax "#702d06"
minus-emph-style              = syntax bold "#702d06"
plus-style                    = syntax "#6aa3a5"
plus-non-emph-style           = syntax "#6aa3a5"
plus-emph-style               = syntax bold "#6aa3a5"
line-numbers-minus-style      = "#702d06"
line-numbers-plus-style       = "#6aa3a5"
line-numbers-zero-style       = "#eeeeee"