feat: overwrite nick color

[casperstorm]

Ability to overwrite nickname colors by providing a hex string.
Note that I didn't add any breaking changes since we are still able to deserde from a string, instead of a map:

The new format looks like

[buffer.nickname.color]
kind = "unique"
hex = "#ff0000" # optional

But we still correctly parse

[buffer.nickname]
color = "unique"

[tarkah]

LGTM. I made one change to simplify the backwards compatible deserialization implementation. Can you please confirm it still works before merging?