Add color configuration options

[srathbone]

I've set the default colors to white so users updating the plugin will have the same colors.

[kristijanhusak]

@bruno- any chance this gets merged in?

[bruno-]

Hey, I don't fully understand why we'd want this feature in this plugin? Can't the user just set the color manually before the #{online_status} interpolation?

[kristijanhusak]

Yes, but it will be same for both offline and online. Idea here is to have 2 different colors for 2 states, like red for offline, and green for online.

[srathbone]

The user could instead do this:-

set -g @online_icon "#[fg=#73d216]✔"
set -g @offline_icon "#[fg=#fc4138]✘"

I'm happy for you to reject the PR but could we put this in the documentation to make it clear to the users how to set custom colours per status?

I could create a new PR to instead add this information to the readme?

[kristijanhusak]

@srathbone nice, i didn't think of that.

I guess we don't need this then, just an example in readme as you said.