Add links to RFC sections, if available

[mdlayher]

This PR extends rfcPat to also match text such as: RFC 3315, Section 17.2, allowing easy linking to various RFC sections.

Here's the output of a quick test program I put together, with the new changes:

[zsh|matt@nerr-2]:~/tmp/gddo 0 % go run main.go
comments: 
// RFC 3315.
// RFC 3315 does some stuff
// RFC 3315 Section does
// RFC 3315 section 7.
// RFC 3315, Section 1.
// RFC 3315, Section 10.
// RFC 3315, Section 1.1.
// RFC 3315, Section 10.12345.
// RFC 3315, Section 10000.200.

output: <p>
// <a href="http://tools.ietf.org/html/rfc3315">RFC 3315</a>.
// <a href="http://tools.ietf.org/html/rfc3315">RFC 3315</a> does some stuff
// <a href="http://tools.ietf.org/html/rfc3315">RFC 3315</a> Section does
// <a href="http://tools.ietf.org/html/rfc3315#section-7">RFC 3315 section 7</a>.
// <a href="http://tools.ietf.org/html/rfc3315#section-1">RFC 3315, Section 1</a>.
// <a href="http://tools.ietf.org/html/rfc3315#section-10">RFC 3315, Section 10</a>.
// <a href="http://tools.ietf.org/html/rfc3315#section-1.1">RFC 3315, Section 1.1</a>.
// <a href="http://tools.ietf.org/html/rfc3315#section-10.12345">RFC 3315, Section 10.12345</a>.
// <a href="http://tools.ietf.org/html/rfc3315#section-10000.200">RFC 3315, Section 10000.200</a>.
</p>
<pre></pre>

And here's an example on a local gddo-server instance: