Proxy: Better errors + remote custom TLS (#1197)

Proxy will be more verbose on errors + possibility to configure custom transport (example: for custom TLS certificates)

middleware/proxy_1_11_n.go

@@ -0,0 +1,13 @@
+// +build !go1.11
+
+package middleware
+
+import (
+	"github.com/labstack/echo"
+	"net/http"
+	"net/http/httputil"
+)
+
+func proxyHTTP(t *ProxyTarget, c echo.Context, config ProxyConfig) http.Handler {
+	return httputil.NewSingleHostReverseProxy(t.URL)
+}