Add support for passing linting statistics in as file or argument

[drewsilcock]

It would be really useful to have support for other methods of inputting the output of the linting statistics. Currently they must be passed in by stdin, however I can see two other potential uses:

• Read from file - useful when piping output between commands is difficult.
• Pass string in directly - useful when generating chart from another python module via import flake8chart as opposed to using as a command-line utility.

[drewsilcock]

(Awesome module by the way, so satisfying to chart your progression when reducing those pesky linting errors!)