Support "section" references like MMark

[ronaldtse]

In normal AsciiDoc a section reference will produce the title.

In RFCs, a section reference should produce something like "Section 5.4.3".

In MMark, references like:

See Section (#header)

Will produce See Section 5.4.3.

[opoudjis]

I will need help with this, @paolobrasolin , since my impression was that Asciidoc didn't quite do this with its xrefstyle, and I could be wrong.

[opoudjis]

This is not relevant to the RFC XML output of the gem: the gem, just like MMark, produces the output <xref target="header"/>. It is xml2rfc that ends up inserting section numbers. (And it does.)