Feature - Add an option to have tags in custom colors defined in the config file.

[b13n1u]

This PR adds a simple option to allow defined tags have custom colors. The tags and their colors can be defined in the config file as described in the documentation. Also a simple example here:

Example:

tag_colors:
  "@high": red
  "@medium": yellow
  "@low": green
  "#urgent": magenta
  "#work": blue
  "#personal": cyan

This would for example show the @high tag in red. I have added a test case for this feature, poe test is happy.

Checklist:

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed. (Not needed for this change)