Add option to configure text color and background colors of panes

[ohcibi]

What problem does this solve?

• The project cannot be used with a light background currently as the text color cannot be changed
• Invidual background colors for panes would add the ability to increase information transmission

How do envision it working?

Note that I use the word 'pane' here to refer to a single "module", or whatever it is called in wtf, that gets displayed with a border. If unclear what I mean please ask instead of assume as I don't already know the wording in this project and I very likely have used the wrong term.

• The text color could be an option similar to the current color.background e.g. color.foreground (if this is already available, consider this issue as a documentation improvement request)
• Styling panes is a bit more of a complex problem. Possible options, I could think of are
  • Text/title matching of text inside the pane or of individual values of the pane
    • would allow something like e.g. using red background for the bamboo pane if the latest job has failed to build
  • Adding something like a class (as in "css class") to the meta information of a pane and match based on that
  • positional colorization (e.g. top row of panes is blue, center row is yellow, bottom row is red)

[jaimejim]

I would personally like some generic mechanism for changing the colors of items within a module. There are some global variables that can be used in the config.yaml, so far I know of:

text, heading, subheading

And the use of ::b to make the text bold.

Not everything seems to be documented on https://wtfutil.com/configuration/global_settings/