Add ability to set arbitrary headers

This is useful if you want to add headers for things like HTTP Strict
Transport Security or HTTP Public Key Pinning.

headers.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"net/http"
+	"strings"
+)
+
+type addheaders struct {
+	h       http.Handler
+	headers []string
+}
+
+func (a addheaders) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	for _, header := range a.headers {
+		headerSplit := strings.SplitN(header, ": ", 2)
+		w.Header().Add(headerSplit[0], headerSplit[1])
+	}
+
+	a.h.ServeHTTP(w, r)
+}
+
+func AddHeaders(headers []string) func(http.Handler) http.Handler {
+	fn := func(h http.Handler) http.Handler {
+		return addheaders{h, headers}
+	}
+	return fn
+}