Add color theme config

[a-kenji]

• add option theme that allows for setting of a theme,
  the default is default

• under themes the themes can be described as follows:

either

themes:
  default:
    fg: [0,0,0]
    bg: [0,0,0]
    black: [0,0,0]
    red: [0,0,0]
    green: [0,0,0]
    yellow: [0,0,0]
    blue: [0,0,0]
    magenta: [0,0,0]
    cyan: [0,0,0]
    white: [0,0,0]
    orange: [0,0,0]

or

themes:
  default:
    fg: 0
    bg: 0
    black: 0
    red: 0
    green: 0
    yellow: 0
    blue: 0
    magenta: 0
    cyan: 0
    white: 0
    orange: 0

If the key is different from default, it needs to either be specified on
start with options --theme [THEME], or in the configuration file under
theme: [THEME].

closes #390

[a-kenji]

This is not quite ready yet, since the theme reverts to its default
once its mode is changed.

I am not sure where we should attach the client attrs in route in order to get the
attributes of the client palette attributes.

I would be happy about any input here.

[imsnif]

Just a heads up: some terminals don't support truecolor. That's the reason our default theme uses 256 colors. It might be a good idea to differentiate the themes to 256 and truecolor themes. Or at least provide a configuration option for 256 colored themes.

[a-kenji]

Oh yeah, that is also possible to specify currently.
I will update the current post.

[kunalmohan]

I am not sure where we should attach the client attrs in route in order to get the
attributes of the client palette attributes.

The idea was to have screen and plugins to store a copy of the palette and generate the mode_info themselves when the mode changes (instead of having the router thread do that). Original comment: https://discord.com/channels/771367133715628073/773128553524494367/843837258120298577

But it'll, probably, be a while before this is done (Brooks is currently working on the resizing algorithm), so maybe go with plan B and store the palette in SessionMetaData for now and use that palette here instead of the default one?

What do you think @imsnif ?

[a-kenji]

I think that is a good idea for now then.
Thank you for showing me the discussion @kunalmohan!