Add an option for file/STDIN to allow for third party chart data

This adds a -i/--input flag that takes a file or STDIN. In concert with something like fakehubstats, this would allow githubchart to create graphs of non-public or even non-git related streaks.

This is basically my first foray into Ruby, so I'm looking forward to feedback. I still need to add tests.
You may want to check "If file exists or (input equals '-' and stdin isn't a TTY", for sanity