Expand rdoc-ref targets at the end of ri output

[st0012]

There have been several document refactors in ruby/ruby that extract individual methods/classes' documentation into separate files, like ruby/ruby#6567

Because RI is not capable of rendering those references, RI users are left with dramatically fewer documentation on those methods/classes.

This commit adds a new option --expand-ref (default: true) to expand all the rdoc-ref targets at the end of the output.

Closes #1097

[adam12]

Tested this on a few core/stdlib methods and it worked well! 👍

[adam12]

One issue that's not directly related but should be worth noting is it fails to work on [#module-JSON-label-Parsing+Options].

I know it doesn't start with rdoc-ref but it appears to be a valid reference at least on the HTML generated site.

(the example is from JSON.parse, by the way).

[zenspider]

Might be valid... but it might need to be changed just to fit this PR. Frankly I don't care as long as ri Array#pack works... But I do agree that xrefs for options NEED to be expanded sooner or later. Getting doco that says **opts and then has a broken xref you can't actually use isn't helping ANYONE.