Preserve only blocks annotated explicitly.

I think we should preserve only blocks annotated explicitly.

Because:
- [Annotating JavaScript for the Closure Compiler](https://developers.google.com/closure/compiler/docs/js-for-compiler) does not say that a compiler/minifier does not preserve the block which have "copyright" words but does not have any explicit annotations.
- In some use-cases, I don't preserve the full text of a license. However, if we include "copyright" word to patterns, esmangle preserves the full text of a license.

How do you think about this?