Allow pyproject.toml to override a global coveragerc

[crazystick]

Is your feature request related to a problem? Please describe.
I maintain a docker image which is used for testing multiple different projects, and I'd like to provide a set of global defaults for coverage but be able to override them per project in pyproject.toml

Describe the solution you'd like
I would like config to be loaded first from a global default file, e.g. /etc/coveragerc which would replace any defaults from CoverageConfig. After this the existing logic which reads the various configuration files (and exits as soon as it finds a valid one) will still work fine. I also believe this implementation will not break existing configurations.

Describe alternatives you've considered
I tried passing config directly on the command line and via a specified config file, but because read_coverage_config breaks as soon as it finds the first valid configuration file, it didn't work.

It could be possible to just remove the early exit in read_coverage_config but that would potentially break people's existing configuration.

Additional context
The main thing I'd like to specify in the global defaults is fail_under - I would like to have a target that is organisation wide, and anyone configuring a different value via a pyproject.toml would be a red flag. Sometimes it's just not practical to hit the target though.