Add support for running script on multiple files at once

[keyan]

It would be helpful to add an argument which allows for running the fixer over an entire directory of .py files recursively.

This would involve adding an optional argument to flask-ext-migrate/startup.py, which currently only accepts a single file argument. Additionally flask-ext-migrate/__init__.py would require an additional method to find files in the specified directory and call fix() on each individually.