Unify naming of .nbautoexport config in code

[jayqi]

There are three names being used for the .nbautoexport config file:

• SAVE_PROGRESS_INDICATOR_FILE
• "sentinel"
• "config"

We should use one name to make the code easier to read. config seems like the best choice.

Separated this from issue #34, because this will be a noisy change for the git diff.