Modules: added ngx.fetch().

Modules: added ngx.fetch().

This is an initial implementation of Fetch API.

The following init options are supported:
body, headers, buffer_size (nginx specific), max_response_body_size
(nginx specific), method.

The following properties and methods of Response object are implemented:
arrayBuffer(), bodyUsed, json(), headers, ok, redirect, status, statusText,
text(), type, url.

The following properties and methods of Header object are implemented:
get(), getAll(), has().

Notable limitations: only http:// scheme is supported, redirects
are not handled.

In collaboration with 洪志道 (Hong Zhi Dao).