Added basic customisation for menubar colors

[V1RE]

#156 looks promising, but hasn't seen much action in the last 2 years. So I
made a quick and simple way to customize the foreground and background colors of the menubar using env vars.

Haven't used rust too much, so lmk if I can improve on anything

[cantino]

Thanks @V1RE! The only problem is if we merge this, we need to support these options even in an eventually, more configurable world. Perhaps thats fine though.

[cantino]

Do you want to go through and wire up other color options too (text, background, highlight)?

[V1RE]

Sure, what env vars should I use?

[cantino]

MCFLY_PROMPT_COLOR, MCFLY_HIGHLIGHT_COLOR, MCFLY_SELECTION_COLOR might be all of them?

[V1RE]

@cantino What to do with the lightmode setting? this would kind of make that obsolete, right?

[cantino]

Can we integrate this with the new configuration system #407?