Add support for custom modes and modelines

[e-tho]

Adds 'custom' boolean to outputs.<n>.mode for non-advertised resolutions and outputs.<n>.modeline option for precise timing parameters.

The modeline option accepts both list and record formats:

• List format enables direct copy-paste from cvt/gtf output

  modeline = [ 373.25  1920 2064 2264 2608  1080 1083 1088 1168  "-hsync" "+vsync" ];

• Record format provides self-documenting configuration with named fields

  modeline = {
    clock = 373.25;
    hdisplay = 1920;
    hsync-start = 2048;
    hsync-end = 2248;
    htotal = 2576;
    vdisplay = 1080;
    vsync-start = 1083;
    vsync-end = 1088;
    vtotal = 1120;
    hsync-polarity = "negative";
    vsync-polarity = "positive";
  };

Closes #1407