Fix regression: non-working direct colors in syntax files (#2252)

Fix regression: non-working direct colors in syntax files (#2252)

After 9ad4437, directly specifying color names (instead of syntax groups)
in syntax files no longer works. In particular *.patch and *.diff files
are not highlighted, since in patch.yaml direct colors names are used.

Restore the previous behavior of GetColor (fallback to direct colors if
no syntax group found) to fix this regression, but also make some changes
in StringToStyle and StringToColor to still fix the issue which was fixed
by 9ad4437. In other words, ensure that there is no confusion between
direct colors ("red", "green" etc) and syntax groups omitted in the
colorscheme file.