[FEATURE] Add support for envreport action to be pluggable

[tonybaloney]

Requesting (and willing to submit a PR) that the plugin system support a hookimpl for the envreport action.

I'm developing a plugin for Pipenv https://github.com/tonybaloney/tox-pipenv using the hook implementation API in 2.9.1.

So far it's working nicely, however as stated in the docs, the hook implementation options are not complete.

Whilst the default option for list_dependencies_command can be overridden at runtime, I want the plugin to be self-contained. Also, setting the configuration option when the plugin ignores it seems a little obtuse.

https://github.com/tox-dev/tox/blob/master/tox/config.py#L595-L599

[tonybaloney]

Raised PR with potential implementation

[asottile]

I think you can do this without needing new apis. For example, here's how tox-pip-extensions sets the install_cmd: https://github.com/tox-dev/tox-pip-extensions/blob/a989ec95ef8f3c5e63bb809308cd4260f0690e76/tox_pip_extensions.py#L71