options: Add `truecolor` to control the mode

[JoeKar]

Currently even 256 color schemes like the default one don't look as defined and then most probably expected due to used terminal palette. So I suggest to determine the true color support based on the already mentioned $COLORTERM environment variable and furthermore the overall control should be able with a micro internal option to prevent the additional definition of a further user defined environment variable.

w/o vs w/ true color support/mode:

This could close #2859 too.