Some extension of plugin

HI, just wanted to have control of what should be compressed dependently on size, also wanted to have option to NOT append .gz extension (usualy CDN can respond with correct headers independently of extension).

Result - 2 options can be passed to plugin:
* append <true> - ability to choose whenewer you want to append .gz extension. Can be set to ```true``` or ```false```.
* threshold <alse> - ability to set minimum size of compressible file (hard limit to 150 bytes). Reason of some compression overhead with small files. Can be set to ```String|Number|Boolean``` - details below.

Actualy tryed to make ```threshold``` option pritty handy - it can receive ```String``` (parsed by ```bytes```, hard limited in 150), Number (exact size in bytes, hard limited in 150), Boolean (false - compress independently on size, true - resulting in set to hard limit - 150).

Also replaced event-stream with through2 - reason is that ev built with Streams1 in mind so during extending of your plugin, I had couple of problems because of incapability of ev with Streams2.

Also rewriten tests - including originals with addition of test suit for extened capabilities.