fix: Adding in missing getter for document.head

[rschristian]

Closes #69

Simply missing a getter for head.

Full stack trace of the error that this fixes. The one in the linked issue is fairly outdated and the error position is no longer correct:

TypeError: Cannot read property 'appendChild' of undefined
    at ../../node_modules/critters-webpack-plugin/dist/critters-webpack-plugin.js:199:23
        at Array.map (<anonymous>)
    at ../../node_modules/critters-webpack-plugin/dist/critters-webpack-plugin.js:195:24
        at Array.forEach (<anonymous>)
    at CrittersWebpackPlugin.embedAdditionalStylesheet (../../node_modules/critters-webpack-plugin/dist/critters-webpack-plugin.js:188:48)
    at CrittersWebpackPlugin.process (../../node_modules/critters/dist/critters.js:633:12)
    at handleHtmlPluginData (../../node_modules/critters-webpack-plugin/dist/critters-webpack-plugin.js:85:14)
    at eval (eval at create (../../node_modules/tapable/lib/HookCodeFactory.js:74:10), <anonymous>:13:1)
    at AsyncSeriesWaterfallHook.eval [as promise] (eval at create (../../node_modules/tapable/lib/HookCodeFactory.js:74:10), <anonymous>:2:8)
    at AsyncSeriesWaterfallHook.lazyCompileHook (../../node_modules/tapable/lib/Hook.js:154:20)
    at ../../node_modules/html-webpack-plugin/index.js:267:70

Adds a couple tests to ensure additionalStylesheets works.

[rschristian]

If there's a slightly less awkward way to create another chunk, certainly let me know. This was just to generate another stylesheet

[rschristian]

Unfortunately this is correct at the moment, and Critters no longer dedupes, see #88

[rschristian]

@developit Sorry to ping, but any chance you could take a look at this? This is the only remaining blocker for upgrading to HtmlWebpack v4 in preact-cli

[developit]

@rschristian published as critters@0.0.15!

[rschristian]

Thanks!