Allow custom named color schemes as plugins.

[dukejones]

This will allow users to keep sets of named color schemes, switch between them, and to share them publicly.

Example: theme-bobthefish-colorschemes

Working version of the Monokai example from the wiki pasted below:

function __bobthefish_color_scheme_monokai -S -d 'Monokai colors'
  __bobthefish_colors base16-dark

  set -l base00 272822
  set -l base01 383830
  set -l base02 49483e
  set -l base03 75715e
...
  set -x color_virtualfish              $base0D $colorfg --bold
  set -x color_virtualgo                $base0D $colorfg --bold
  set -x color_desk                     $base0D $colorfg --bold
end