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 make this and the preceding check one larger block. You can also use "fail "string"" to do the error message and exiting, so you could do something like:

```
if input.eql? '-'
  fail 'No data provided on stdin' if STDIN.tty?
  contents = STDIN.read
else
  fail 'file does not exist' unless File.exists? input
  contents = File.read input
end