Don't serve requests with incorrect or empty Host header

[rmrio]

Generally is not a good idea to serve all requests on the blog IP even with empty or unknown host header. The good practice is to serve requests with correct 'Host' header and return 400 otherwise.
See rfc2616 for details.

[kabukky]

Hi rmrio,
thanks for your PR. Could you submit it again against the development branch? The master branch is just updated with releases. I will update the readme to reflect this.

A couple of things:

• Do we really need a new dependency for this?
• How much overhead does this create for every request?
• Journey has a "pages" feature (see serving standalone projects with customized url #18 ) that lets users host static pages. Will this change allow redirecting different domains to journey pages (e.g. www.mygreatproject.com to www.myjourneyblog.com/pages/greatproject/)?