Added an option preferred encodings array

[danielgindi]

Will be used in expressjs/compression#172

[danielgindi]

@dougwilson Would you mind adding a minor change to accepts if/when this PR is merged? This will save some time. Then I'll revert the brotli branch to use accepts module with a simple extra argument.

[nicksrandall]

@dougwilson thanks for shepherding this along.

[wesleytodd]

After merging this I was looking at the api and realized we are limiting ourselves by passing an array directly, so I wrapped it in an options object so we have less of a chance of needing to break that api in future changes. That is what I will release with 1.0.