Adding new highlight group

1. Add it to src/ui/colors.h.
2. Compile errors will point to arrays and switch-statements that need to be updated (need -Werror, so configure with --enable-developer or with --enable-werror).
3. Make use of the new highlight group.
4. Add it to data/man/vifm.1 (don't forget to update modification date).
5. Add it to data/vim/doc/app/vifm-app.txt (don't forget to update modification date).
6. Add a "test" to tests/test-data/syntax-highlight/syntax.vifm.
7. Add it to data/vim/syntax/vifm.vim (don't forget to update modification date).
8. Running scripts/check-syntax-changes should show changes in highlighting for new lines.
9. Remove an entry from the TODO file if one exists.
10. Add to the ChangeLog file.