add support for link preload hrefs

[akrawchyk]

This PR adds support for rel="preload" stylesheet links as specified here https://w3c.github.io/preload/.

It's useful for implementing the preload pattern with loadCSS.

[akrawchyk]

Note, this depends on addyosmani/oust#19. I've updated the package.json with the repo for the changes to the oust package as an example.

[akrawchyk]

These tests fail with the current master on node v6.2.1 branch as well:

1. CLI acceptance should work well with the critical CSS file passed as an option
2. CLI acceptance should work well with the critical CSS file piped to critical
3. CLI acceptance (remote) should generate critical path css from external resource

Seems to be related to nodejs/node#3389 since the data returned from the call to execFile is a Buffer.

[akrawchyk]

I believe this is the issue causing failing tests nodejs/node#6666.

Seems from https://github.com/nodejs/node/blob/357f904169c39bbc9c2d8558bbffce2337c83c1b/lib/child_process.js#L153 the buffer is not recognized as the expected encoding.

To workaround, I've casted the output from stdout as strings. If the output is already a string, it will remain a string, but if it's a buffer, it'll be converted to a string.

[addyosmani]

Can you reset this? oust 0.3.0 is now out with the PR you submitted.

[akrawchyk]

So I'm wondering if #129 is a better idea. I'm pleased oust supports preload links, but I noticed inline-critical automates the preload pattern.

This still might be useful, however, if there are manually added preload links in the HTML that critical will parse. Perhaps both PR's make sense?