teach add -f option for ignored submodules

When the user wants to bypass the ignored status configured by
submodule.<name>.ignore=all it is now allowed by using the -f option.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Merge remote-tracking branch 'hvoigt/hv/fix_add_ignored_submodule' into hv/fix_add_ignored_submodule