When I specify extractors, I want to specify only the extractors I want to override

[ahus1]

Describe the current behavior

When specifying an extractor specific for one file type, but no extractor for the other file types in the project, I get an exception "Cannot read property 'extractor' of undefined"

To Reproduce

1. use html and js as content
2. add an extractor for html, but not js
3. run purgecss
4. see error

Expected behavior

I want the default extract to be used for any file type that I don't specify. Apparently this happened to other people previously as well: #95

Desktop (please complete the following information):

• OS: Windows (but shouldn't be related)
• Version of Purgecss: 1.1.0

Additional context

Suggested fix: in getFileExtractor() the DefaultExtractor should be returned when extractorObj is undefined

purgecss/src/index.js

Lines 230 to 237 in 9b637bd

	getFileExtractor(filename: string, extractors: Array<ExtractorsObj> = []) {
	if (!extractors.length) return DefaultExtractor
	const extractorObj: any = extractors.find(extractor =>
	extractor.extensions.find(ext => filename.endsWith(ext))
	)
	return extractorObj.extractor
	}

[Ffloriel]

Yes, it would definitely be better if you can specify only the extractors I want to override.
Aiming at changing this for the next release.