Fix interaction with other plugins

[rluba]

Proposed changes

Currently, PurgeCSS purges at the beginning of tree traversal. If you use other plugins that transform selectors, PurgeCSS purges them before the other plugins had a chance to transform them.

One example where this fails is the following interaction with postcss-nested:

.selector {
    &:hover {
    }
}

PurgeCSS gets rid of &:hover before postcss-nested can transform it to .selector:hover, which would have been kept.

This PR changes the behavior so that PurgeCSS purges after all other transformations have been applied.

Types of changes

What types of changes does your code introduce?

• 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!