Add --static-dir server startup flag

[jisantuc]

Improvement

If you want to build a UI on top of Franklin, it would be cool if you could just tell Franklin about that and then serve everything from the same place. As is you'd need to set up nginx or something. This could be supported via a --static-dir flag that creates an http4s static service of whatever you pass in. This could also be used to override the existing UI that we've about 10% implemented if people have their own plans and/or don't feel like waiting for us.