Add flag to bind on selected host, instead of default "0.0.0.0"

So essentially, in the current codebase, hakyll defaultly binds on "0.0.0.0", which it listens on *all* network interfaces.

I think that a much saner default is to bind *only* to localhost, and give the user the ability to override that, if needed (with a newly added "-h"/"--host" command line arg). For the deprecated "./site preview" mode, I left the default at "0.0.0.0" for the sake of backwards compatibility.

I think that binding on every interface defaultly is a definitely not a good idea. Principle of least privilege and all that.