Allow specifying project_dir from within a --config file

[Julian]

I'm only a day or two into using the project, which seems quite nice, so apologies if I'm missing this:

Is your feature request related to a problem? Please describe.
It seems there isn't a way to specify where the root of a project lives, and openapi-python-client assumes that cwd is the parent of where it lives. That makes it a bit awkward to execute openapi-python-client update (it means one basically needs to do so from a directory up from where I'd expect) once you already have a git repository with a generated client that you're maintaining.

Describe the solution you'd like
One or both of:

• assume that the project_dir is relative to --config when provided (i.e. that someone has placed the --config inside their project_dir, so start there)
• add a project_dir option to config files themselves which specifies, relative to the file itself, where the project lives (and when not present, default to cwd I suppose as is assumed now).

Describe alternatives you've considered
Only doing one or the other of the above.

Thanks again!

[dbanty]

update is being removed, and instead --overwrite and --output-path are being added to generate. I think this solves this request.