Prefer zlib over zopfli for gzip?

There was some nice work done in #36 to make iltorb and node-zopfli-es optional.

But zopfli-compat.js is preferring zopfli over zlib, and printing a warning if node-zopfli-es is not installed. zopfli may create gzip assets 5% smaller than zlib, but it is much slower (~100x).

Since shrink-ray-current is for use at runtime, speed is very important and I would prefer to use zlib not zopfli. This would be consistent with brotli-compat.js which is preferring zlib over iltorb. Would you be open to this change, or alternatively silencing the warning Module "node-zopfli-es" was unavailable?

[keeganstreet]

Hi Alorel, that’s OK. If we add an option the change might look something like this:

• Add a boolean option called useZopfliForGzip which defaults to true
• In index.js, move const zopfli = zopfliCompat(); inside the compression function so we can pass opts.useZopfliForGzip to zopfliCompat.
• In zopfli-comat.js, return require('zlib') if useZopfliForGzip is false.

Would you like a PR for this?

[Alorel]

Thanks for the contribution, published as 4.1.0