Add reverse-proxy functionality

[mufeedvh]

@luciusmagn suggested this feature, Thank You! 🙌

i.e. rerouting domains and routes to different ports on the machine

Multiple domains are not handled at the moment, I will probably think about adding this.

Implementation

Here is how I am planning to implement this feature.

The configuration will look like this, just use a hostname:port combination instead of static file route to configure. simple.

"routes": {
    "/": "index.html",
    "/example": "example.html",
    "/blog": "blog.example.com",
    "/staging": "blog.example.com:8080"
},

While checking for routes, it reads the file (NamedFile()) and responds with the corresponding Content-Type at the moment, this feature should require a check for hostname/file validation before the response.

SIDE NOTE: This might be expensive in performance, in-memory/cached handling of the configuration would be a workaround?