http2: addtl http/2 settings

Currently, node.js http/2 is limited in sending SETTINGs,
that are currently implemented by nghttp2.
However, nghttp2 has the ability to send arbitary SETTINGs,
that are not known beforehand.
This patch adds this feature including a fall back mechanism,
if a SETTING is implemented in a later nghttp2 or node version.

Fixes: nodejs#1337

lib/internal/http2/core.js

@@ -946,7 +946,7 @@ function pingCallback(cb) {
 // All settings are optional and may be left undefined
 const validateSettings = hideStackFrames((settings) => {
   if (settings === undefined) return;
-  assertIsObject(settings.customSettings, 'customSettings', 'Number');
+  assertIsObject.withoutStackTrace(settings.customSettings, 'customSettings', 'Number');
   assertWithinRange.withoutStackTrace('headerTableSize',
                                       settings.headerTableSize,
                                       0, kMaxInt);