feat: implement try_rgb_from_str
feat: implement try_rgb_from_str

We can now convert strings to `ansi_term::Color::RGB`s via the
`try_rgb_from_str()` method. As long as the input is a sequence of
three `u8`s, separated by commas (such as "0, 0, 0"), we can create
the corresponding `Color`. Malformed inputs will return an error.

ben01189998819991197253/contrail