Add option blocklist to webpack plugin

[NelsonReis]

Proposed changes

blocklist option wasn't available for use on webpack plugin, although it's on the base purgecss code.
Solves #609 (that I've also created).

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)

Further comments

I guess this is simple enough.