Use Host header to build urls

[srstsavage]

Currently urls are built using localhost unless --api-host is explicitly passed. It would be nice if franklin inspected the Host header of incoming requests and used the value (if present) as the domain in the constructed url. This would allow a single franklin instance to be hosted under multiple domain names, among other deployment scenarios.

[srstsavage]

Checking X-Forwarded-Host would be prudent as well

[jisantuc]

Hey @shane-axiom, thanks for the issues! Just a heads up I probably won't be able to get to these until at least two weeks from now since close to 100% of my extra time right now is going toward preparing for a FOSS4G workshop I'm giving. Do you know Scala? If so I'd be happy to a review a PR that makes this change. Also feel free to drop by the Gitter channel if you run into any trouble

[srstsavage]

No worries, I'm not blocked on these particular issues, they're more nice-to-have/best practices kind of requests. Appreciate the response and the project!