[Feature request] Default underline color

[tudurom]

I would like to specify bar's default underline color as a parameter, something like:

lemonbar -g "widthxheight+x+y" -f ...skip... -U #65737e

Any interest?

[jeromebon]

@LemonBoy Are you interested in this? If it's the case I can prepare a PR.
This would be easy to implement by reproducing what's already done with -F and -B

[LemonBoy]

Sure thing, let's do this for consistency sake.