interpret-trailers: add options for actions

interpret-trailers: add options for actions

Allow using non-default values for trailers without having to set
them up in .gitconfig first.  For example, if you have the following
configuration

     trailer.signed-off-by.where = end

you may use "--where before" when a patch author forgets his
Signed-off-by and provides it in a separate email.  Likewise for
--if-exists and --if-missing

Reverting to the behavior specified by .gitconfig is done with
--no-where, --no-if-exists and --no-if-missing.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>