Allow multiple transform streams

[jakepruitt]

• breaking change to the API that forces the transform property to be
  an array
• will require a major version bump and changes downstream

@rclark for the review. Unblocks migration and validation in a tilelive-copy operation.

[rclark]

Thought about this a little more this morning, and as I understand, the primary issue you are running into is the instanceof check here.

We could unblock you in a non-breaking fashion by just dropping this check. It would allow streams built by tools like stream-combiner to be passed as tilelive.copy transforms.

The downside is that a caller who drops in something that can't act like a transform stream will fail with a less focused error message, but I think that's worth the tradeoff of leaving the API unbroken.

[coveralls]

Coverage increased (+0.008%) to 97.685% when pulling 8994920 on multiple-transforms into 0e56c60 on master.

[jakepruitt]

@rclark okay, closing for now then