Implement section-signifier similiar to {chapter,part}-signifier

[vvavrychuk]

{chapter,part}-signifier work only for book. I would like to have similiar feature when doing a small article. Also, {chapter,part}-signifier could be set only globally. For section-signifier would be convinient to set it locally too:

= Document

== Some section

....

== Variants/Examples
:sectnums:
:section-signifier: Variant/Example

=== Foo
=== Bar