Add predefined color palettes

[Yaraslaut]

Refs #1285

PR adds few predefined color palettes, values are taken from https://github.com/Gogh-Co/Gogh/tree/master/themes
and supported themes are:
contour, monokai, one-dark, one-light, gruvbox-light, gruvbox-dark, solarized-light, solarized-dark, papercolor-light, papercolor-dark.

in config user can specify both single colorscheme or as a map like:

colors: contour

or

colors:
    light: monokai
    dark: contour

[christianparpart]

Sounds really good. We however also may want to mention this in the docs/ - website. :)

[christianparpart]

Thank you :)