Adds support for more commands and piping gs output to a writable stream

Hiya,

Thanks for open sourcing your ghostscript connector, I made a few alterations to the fork we're hoping to use in our new Image Processor and thought I'd pull them back if you'd like them (adhering to your existing style in the repo.)

They're a bunch of commands we needed and support for piping the response from ghostscript to a writable stream. I've also updated the README.md although I didn't write any tests for the lib, I might be able to peel some off the project that embeds it though.

Thanks again!

- JLS