add dependencies referenced by posthtml-include

[jdanyow]

fixes #488
closes #1226

🚨 note: works with config like this:

parcel/test/integration/posthtml-assets/.posthtmlrc.js

Lines 1 to 7 in e7df99d

	module.exports = {
	plugins: {
	"posthtml-include": {
	root: __dirname
	}
	}
	};

not like this:

parcel/test/integration/posthtml/.posthtmlrc.js

Lines 1 to 7 in e7df99d

	module.exports = {
	plugins: [
	require('posthtml-include')({
	root: __dirname
	})
	]
	};

[devongovett]

What do you think about making this less specific to posthtml-include? Could we just pass this function to all posthtml plugins?

[jdanyow]

Sounds good to me, I've update the PR.

[devongovett]

cool, looks like the test is failing though

[jdanyow]

hmm- will rebase and try again... passing for me locally.

[jdanyow]

ok rebased- it's passing now. There's a vue test that's failing though: "should produce a vue bundle using preprocessors". I think it broke in #1375