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

---

**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.
Maybe a better implementation would be to make a link mapping (coding in the browser, caveat reader)

```ruby
LINKSET_METADATA_MAP = {
  wiki: 'wiki_url',
  docs: 'documentation_url',
  mail: 'mailing_list_url',
  code: 'source_url',
  bugs: 'issue_tracker',
}
def ....
  update_attributes!(linkset_from_spec(spec))
end
def linkset_from_spec(spec)
  {
  home: spec.homepage,
  }.merge(
    Hash[
      LINKSET_METADATA_MAPPING.map { |attribute, metadata_key|
        # metadata will always be a hash `{}` unless we erroneously allow it to be something else in the setter
        if link = spec.metadata[metadata_key]
          [attribute, link]
        end
      }.compact
    ]
  )
end
```

I also would like some discussion on standardizing metadata keys and format before merge.  I think I'd prefer a metadata key 'urls' that has all the mappings inside it, e.g.

```ruby
{ urls: { docs: docs_url, wiki: wiki_url }}
```