fix: make header color configurable

[jderuiter]

The header template (TODOFISH_HEADER) was parsed before the config file was sourced, so the default color was always used.

[hugokernel]

If you want to override the config like TODOFISH_HEADER, you need to use the -c argument and the config file will be loaded after.

[jderuiter]

Actually I didn't want to override TODOFISH_HEADER, but I wanted it to respect the COLOR_TITLE that I set in my config. It kept being bright green, despite setting COLOR_TITLE to something else. If TODOFISH_HEADER should also be configurable I will need to add an additional check to see whether it was overridden in by the config file.