Support updating for files generated from model-generate

[tkutcher]

Currently, to get any updates to a QuickBase table in the python classes you'd have to re-run model-generate which would overwrite the entire file.

Current workaround to not lose code added to generated models files is to refactor (methods, etc.) in to a separate module and you can pass the relevant QuickBaseTable as needed.

Another issue is if a field label changes then the class attribute would get overwritten potentially breaking code.

Could introduce an update command which instead of straight writing to a file, reads from the file it is going to update and only inserts new fields/reports/etc. Fixes first issue, and second issue could be addressed by just having both fields there in the class then user can manually delete the one they don't want + fix referring code.

[tkutcher]

With #40 there is now some support for updating specific files. Will keep this open for an even smarter "update" if need be.