add media option for responsive preloading

[decrek]

<link> elements have the ability to accept media attributes. These can accept media types or full-blown media queries, allowing you to do responsive preloading.

I've added the media option to pass the value for the media attribute, like so:

plugins: [
  new HtmlWebpackPlugin(),
  new PreloadWebpackPlugin({
    rel: 'preload',
    media: '(min-width: 600px)'
  })
]

Fixes #69

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[decrek]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[decrek]

@jeffposnick The project is not updated in a while, probably because it is was sort of feature complete, but since then I think some things changed regarding Travis. Could you look into why Travis is failing?

[jeffposnick]

Hey @decrek—Thanks for this submission, and yes, it would be good to get this functionality in.

There was a fairly substantial rewrite to the codebase that was just checked in to master, so unfortunately this PR now has a number of merge conflicts.

If you're still up for contributing the code, the new place to add in that code would be somewhere around

preload-webpack-plugin/src/index.js

Lines 68 to 71 in 082beca

	const attributes = {
	href,
	rel: options.rel,
	};

(and then another corresponding change to the test spec.)

If you'd prefer, though, I'm happy to force-push the updates to your PR myself and save you the additional work.

[jeffposnick]

I've got similar changes in https://github.com/GoogleChromeLabs/preload-webpack-plugin/compare/media that is compatible with the current master.

I'm still happy to force-push them into your PR branch if you'd like "official" attribution for the original contributions you've made. (Or you could rebase your PR against that branch.) Alternatively, I can just PR in my changes if you'd rather not bother.

[decrek]

Hi @jeffposnick I dunno, I tried to to this but I dont see my commit anywhere anymore. But anyways it's all good, glad it gets added