function createStream( transform, flush ) {
		opts.transform = transform;
		if ( arguments.length > 1 ) {
			opts.flush = flush;
		} else {
			delete opts.flush; // clear any previous `flush`
		}
		return new Stream( opts );
	}
Creates a transform stream.

@private
@param {Function} transform - callback to invoke upon receiving a new chunk
@param {Function} [flush] - callback to invoke after receiving all chunks and prior to the stream closing
@throws {TypeError} must provide valid options
@throws {TypeError} transform callback must be a function
@throws {TypeError} flush callback must be a function
@returns {TransformStream} transform stream