Add ability to set authenticated user in request header (#889)

* Add ability to set authenticated user in request header

Some web applications provide the ability to authorize users based on
the authenticated from Basic Auth. This patch provides a way to set a
key to which the authenticated user can be set in the Header.

For example, if I set `HeaderValue = "X-WebAuth-User"` and authenticate,
my application will be able to read my user name from that header and
provide me with the proper access.

This fixes #802

types/types.go

@@ -222,8 +222,9 @@ type Cluster struct {
 
 // Auth holds authentication configuration (BASIC, DIGEST, users)
 type Auth struct {
-	Basic  *Basic
-	Digest *Digest
+	Basic       *Basic
+	Digest      *Digest
+	HeaderField string
 }
 
 // Users authentication users