Add config option for selected text color setting

[Luxcium]

as per: Add a config option for selected text colour setting #2934

Not really useful you may think, I have put a "commented out" reminder within a TODO memo, to remember to do it I will try to change it myself but until then it could stay there as a reminder

• To help whoever reviews this PR, it'd be extremely helpful to list whether this PR is ready to be merged:
  it is only a commented TODO, for now at least, so it is ready to be merged indeed

• If your PR changes some API, please make a PR for hyper website too: https://github.com/zeit/hyper-site.
  If I can change it myself without breaking everything I will add the option in the list on the web site.

[Luxcium]

I am inside 'xterm.js/src/renderer/Types.ts' and it seems that there is no foreground colour for a selection:

export interface IColorSet {
    foreground: IColor;
     background: IColor;
    cursor: IColor;
    cursorAccent: IColor;
    selection: IColor;
    ansi: IColor[];
}

[Luxcium]

This issue must be resolved at the xtermjs/xterm.js level instead

[blakeperdue]

@Luxcium this was added to xterm.js in xtermjs/xterm.js#3813 , can we now add a config option (like selectionForeground) to Hyper to allow this color to be configured?