Add customizable vars

g:colorminder_last_colorscheme_file
  path to file to save current color scheme
  Default: ~/.vim/.colorminder_last_colorscheme

g:colorminder_scheme_default_gui
  default scheme to use in gui if last_colorscheme_file not found.
  Default: none - uses Vim's default color scheme

g:colorminder_scheme_default_term
  default scheme to use in term if last_colorscheme_file not found.
  Default: none - uses Vim's default color scheme