Derive hostname from Metalsmith metadata if available

[codeguy]

Rather than require a hostname opt, can you also look and see if it is available from the metalsmith metadata? This way I can define a hostname from an earlier plugin.

Affected lines:

https://github.com/ExtraHop/metalsmith-sitemap/blob/master/lib/index.js#L48-L50

Happy to send a pull request if you authorize the change.

[njbraun]

Sounds like a great feature @codeguy ! If you submit a PR, we can take a look.

Thanks!