Function for path to evaluate on each compilation

[SebastianS90]

Proposed changes

Assume you run webpack --watch and have some glob pattern specified as paths for purgecss-webpack-plugin. Now you add some files that match the specified glob pattern and require them from existing files. Webpack happily detects the change in existing files and recompiles, thereby also including the new files we just added.

But purgecss does not yet know about those new files and might strip some selectors that are now in use.

My change allows to pass a function as paths. The function will be evaluated on each compilation, therefore catching all new files in the next --watch run.

Similarily, I added function support for whitelist and whitelistPatterns. This is useful in conjunction with purgecss-whitelister.

Types of changes

What types of changes does your code introduce to purgecss-webpack-plugin?

• 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]

LGTM
Thanks for the PR.