- Add new module `topydo.lib.Colors` and class `Colors` for handling
  output colors and their ANSI codes.
- Create new config options with colors for priorities, projects,
  contexts, metadata, and links and place them under "colorscheme"
  section. When certain config option is absent default color code will be
  applied.
- Empty values:
    ! priority_colors will fall back to NEUTRAL_COLOR
    ! project_color, context_color, link_color, metadata_color will
      return empty string, so priority_color (if configured and applicable) of
      specific item should be used
- Return NEUTRAL_COLOR if supplied color number is out range
- Support user-friendly color names in config. At least for 16 base
  terminal colors.