Use an ignore file to avoid regenerating pieces instead of a separate `update` command.

[dbanty]

Is your feature request related to a problem? Please describe. Several folks have indicated issues that could be solved by instructing openapi-python-client to ignore files. This could also greatly simplify the interface since the separate update command wouldn't be necessary, you'd just tell it to not update the files you don't want it to (e.g. README.md).

Describe the solution you'd like Similar behavior to the openapitools generator where a .gitignore-style file will be automatically read from an existing directory and respected.