Support multiple base configs

[lebauce]

Allow specifing the --base-config option multiple times.
The base-config option is really useful to avoid duplication but, in a complicated setup,
we often end up having multiple base-config files with a lot of duplication between these files.

PR Type

• Feature Pull Request

[decentral1se]

Implementation looks legit.

I don't even want to know why you need multiple base configs 🏃‍♂️

[tadeboro]

We also have a quite complex setup and would probably benefit from this change.

That being said, there is one area that definitely needs improvement in this PR: documenting each base config file's priority. Right now, help text only states that we can use multiple base config files, but it does not specify what happens if two of them contain the same configuration options with different values. Reading through the implementation, the last one wins, but I do not know enough about click to be certain if argument parsing preserves the order of options.

I added one comment about the default value that needs to be resolved before merging.

[lebauce]

@tadeboro The argument parsing preserves the order of options. Regarding the documentation, I tried to improve it but I'm far from being even a medium technical writer so please check if it makes any sense :-)

[tadeboro]

Thank you, @lebauce, for your contribution!