Add support for overriding the default extractor

[adamwathan]

Proposed changes

This PR implements the ability override the default extractor as proposed in #180.

It allows the user to set the default extractor using a new defaultExtractor option that accepts either a class or a function:

// Using a class
new Purgecss({
    content: [`${root}special_characters/special_characters.js`],
    css: [`${root}special_characters/special_characters.css`],
    defaultExtractor: class {
        static extract(content) {
            return content.match(/[A-z0-9-:/]+/g)
        }
    }
})

// Using a function
new Purgecss({
    content: [`${root}special_characters/special_characters.js`],
    css: [`${root}special_characters/special_characters.css`],
    defaultExtractor: content => content.match(/[A-z0-9-:/]+/g),
})

This simplifies configuration if the end user wants to use a custom extractor for everything, as they don't have to provide a list of matching extensions. It also helps to avoid issues where the user wants to use a custom extractor for everything, but forgets to update the extension list after introducing a new file type to the project. Since Purgecss is often only run in production builds, this can help reduce end-user mistakes that cause bugs in production that weren't detected in development.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[Ffloriel]

Thanks!