Project name link should default to source_url.

Currently the generate docs project name links to Elixirs homepage. My expectation was the  link would go to the source_url url. 

So I suggest if the  source_url is set use that otherwise link to Elixirs homepage.

## Further work:

A couple of thoughts beyond this initial change:

  * I'm not sure if the default link to elixirs homepage was intentional. If its not then the default should be no link at all.
  * Linking to the source_url is not the current behaviour of Elixirs docs. 

I suggest we add to the config:

:homepage_url

And then the logic for the title could become:

```elixir
config.homepage_url || config.source_url || ""
```

Happy to implement all that if it seems sensible.

Thanks!