Extend RFC regex to support multiple subsections

[mdlayher]

My previous regex did not create links when multiple subsections were present, e.g. "RFC 4291, Section 2.5.1", as shown here (notice the trailing '1' is not highlighted):

This cleans up and extends the regex to support zero or more subsections, as shown after the patch:

Here is example output from this patch:

[zsh|matt@nerr-2]:~/go/tmp 0 % go run main.go 
2015/06/29 14:39:16 in: 
// foo
// RFC foo
// RFC 4291
// RFC 4291 section 1
// RFC 4291 section 10000
// RFC 4291, Section 2
// RFC 4291, Section 2.5
// RFC 4291, Section 2.5.1
// RFC 4291, Section 2.5.1.1.1.1

out: <p>
// foo
// RFC foo
// <a href="http://tools.ietf.org/html/rfc4291">RFC 4291</a>
// <a href="http://tools.ietf.org/html/rfc4291#section-1">RFC 4291 section 1</a>
// <a href="http://tools.ietf.org/html/rfc4291#section-10000">RFC 4291 section 10000</a>
// <a href="http://tools.ietf.org/html/rfc4291#section-2">RFC 4291, Section 2</a>
// <a href="http://tools.ietf.org/html/rfc4291#section-2.5">RFC 4291, Section 2.5</a>
// <a href="http://tools.ietf.org/html/rfc4291#section-2.5.1">RFC 4291, Section 2.5.1</a>
// <a href="http://tools.ietf.org/html/rfc4291#section-2.5.1.1.1.1">RFC 4291, Section 2.5.1.1.1.1</a>
</p>

[mdlayher]

/cc @adg

[mdlayher]

Hi @adg , any thoughts?

[adg]

Sure, why not.