Adding option to display graph in Quick Look and avoid output to STDOUT

Hi,

I thought this was a great idea for a utility, thanks! I wanted to avoid file cleanup and a manual call to "qlmanage" or "open" in my own process, so I added an option (-p) that runs Quicklook on a temporary file instead of printing to STDOUT. It's backwards compatible with the existing command usage.

No worries if you don't want this functionality, but it was useful for me.

(I deleted my initial pull request and added this one in order to include documentation in the README file)

Thanks again.