Exclude certain assignment files from being overwritten during autograding

[jhamrick]

Fixes #940 by adding a configuration option that allows you to exclude certain files from being overwritten by the source version during autograding. The way you would do this would be like so in nbgrader_config.py:

c = get_config()
c.Autograde.exclude_overwriting = {"ps1": ["file_to_exclude.py"]}

cc @yangkky