code: func (r *Route) Host(tpl string) *Route {
  	r.err = r.addRegexpMatcher(tpl, regexpTypeHost)
  	return r
  }
docs: // Host -----------------------------------------------------------------------
  // Host adds a matcher for the URL host.
  // It accepts a template with zero or more URL variables enclosed by {}.
  // Variables can define an optional regexp pattern to be matched:
  //
  // - {name} matches anything until the next dot.
  //
  // - {name:pattern} matches the given regexp pattern.
  //
  // For example:
  //
  //     r := mux.NewRouter()
  //     r.Host("www.example.com")
  //     r.Host("{subdomain}.domain.com")
  //     r.Host("{subdomain:[a-z]+}.domain.com")
  //
  // Variable names must be unique in a given route. They can be retrieved
  // calling mux.Vars(request).