[feature] Exclude sections from being mangled

[gwillz]

This could be using comments like:

/* @no-mangle */
.protected-rules-here {
  ...
}
.another-protected .rule {
  ...
}
/* @free-mangle */

And also import rules:

@import "draft-js"; /* @no-mangle */

The naming convention could be whatever. There's plenty of good examples, particularly in linters.

For me this is a nice-to-have at the moment, but could be a requirement for bigger projects. So I might submit a PR eventually.