Added keepProxy to not rewrite the URL.

[mcollina]

Fixes #23.

@clarkorz can you check if this works for you?

[clark0x]

@mcollina This works for some cases. But I prefer a rewritePrefix option, works for more cases:

   const rewritePrefix = opts.rewritePrefix || '';
   ...
   let dest = request.req.url
   if (this.basePath !== rewritePrefix) {
     dest.replace(this.basePath, rewritePrefix)
    }
   reply.from(dest || '/', opts.replyOptions)

[mcollina]

Check it out!

[clark0x]

@mcollina LGTM