Add optional directives for Link headers

[SamyPesse]

The W3C spec defines some directives such as nopush: https://www.w3.org/TR/preload/#server-push-http-2

And some CDNs (such as Fastly) supports it to control the behaviour of the CDN (they also support other directives such as x-http2-push-only).

This PR adds an option directives to define these directives:

express()
  .use(netjet({
    directives: ['nopush']
  }))
  .use(express.static(root))
  .listen(1337);

[coveralls]

Coverage increased (+0.04%) to 96.429% when pulling 4812167 on SamyPesse:directive-nopush into e1a09af on cloudflare:master.

[coveralls]

Coverage increased (+0.04%) to 96.429% when pulling 4812167 on SamyPesse:directive-nopush into e1a09af on cloudflare:master.

[coveralls]

Coverage increased (+0.04%) to 96.429% when pulling 4812167 on SamyPesse:directive-nopush into e1a09af on cloudflare:master.

[coveralls]

Coverage increased (+0.04%) to 96.429% when pulling 4812167 on SamyPesse:directive-nopush into e1a09af on cloudflare:master.

[coveralls]

Coverage increased (+0.04%) to 96.429% when pulling 4812167 on SamyPesse:directive-nopush into e1a09af on cloudflare:master.

[coveralls]

Coverage increased (+0.04%) to 96.429% when pulling 93e647c on SamyPesse:directive-nopush into e1a09af on cloudflare:master.

[terinjokes]

Thanks @SamyPesse, sorry I didn't see the notification until now. Both the Preload specification and Fastly's documentation refer to these as "attributes". How did you decide on "directive"?

[SamyPesse]

No worries :)

I've focused on this sentence:

If used with the nopush directive
https://docs.fastly.com/guides/performance-tuning/http2-server-push

But you are right that attributes may be a better name (like the DOM attributes), I'll make the change.

[SamyPesse]

@terinjokes done :)

[terinjokes]

How does 7900bd5 look?

[SamyPesse]

👍

[terinjokes]

Merged as 7900bd5. Packaging it up as 1.3.0 now.