httpserver: add extauth plugin directive (#2444)

caddyhttp/httpserver/plugin.go

@@ -654,6 +654,7 @@ var directives = []string{
 	"mime",
 	"login",     // github.com/tarent/loginsrv/caddy
 	"reauth",    // github.com/freman/caddy-reauth
+	"extauth",   // github.com/BTBurke/caddy-extauth
 	"jwt",       // github.com/BTBurke/caddy-jwt
 	"jsonp",     // github.com/pschlump/caddy-jsonp
 	"upload",    // blitznote.com/src/caddy.upload