Deprecate the `noEtag` option

[EvanHahn]

As discussed in #9, noEtag basically doesn't work. That's because many Express features set an ETag, even if it's been removed.

We need to:

1. When noEtag is set, show a deprecation warning. We should do this as we've done in Helmet before.
2. Remove noEtag from the readme, and any references to ETags.