Add support for gRPC-Web using Envoy's grpc_web filter

For more information, please refer to
https://www.envoyproxy.io/docs/envoy/v1.9.0/configuration/http_filters/grpc_web_filter#config-http-filters-grpc-web

Thanks to @rotemtam for the original implementation with the gRPC HTTP/1.1
bridge in #1201 for making this so trivial for me to contribute!

Implements feature request #456.

reference/modules.md

@@ -81,6 +81,10 @@ config:
   #   methods: POST, GET, OPTIONS
   #   ...
   #   ...
+
+  # This enables gRPC-Web; a bridge to have Ambassador translate between
+  # the gRPC-Web protocol and a native gRPC upstream server.
+  # enable_grpc_web: true
 ```
 
 ### `enable_ivp4` and `enable_ipv6`