Leverage metadata to populate gems urls

Lets have keys for the common urls we associate with ruby gem projects per #718

The README has defined `issue_tracker` as an example metadata entry. So I went with that.
There were a few examples in the wild with the source code defined as `source_url` - and other seemed to like this as well. It is unfortunate that these two are not consistent.

The others (`wiki_url`, `documentation_url`, and `mailing_list_url`) were my best stab. Don't think anyone really had much of an opinion. We can remove from this PR if you want. I added it since those fields are available in `rubygems.org`.

Thanks

/cc @qrush @fwolfst

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**UPDATE:**

Wanted to include my rationale.

Most people don't know you can add urls in rubygems.org.
Personally, I had thought the other gem authors had fancier rubygems pages because they knew more about the gemspec.
So I looked at their gemspec, and even rubygems source just to find out there was a rubygems web UI. Who knew?

Since there are questions on the mailing list and gh issues, I realized I'm not the only one confused by this. Also, all of the data in rubygems is from the gemspec save these 4 fields. Seems like a good candidate for moving these fields into the gemspec as well.
I would argue that is the correct behavior.  It's really irritating the pull down gem spec data over the API and then do something with the homepage url, only to discover it's something like that.  I mean, your example input really isn't a url...