Feature request: allow fallbacks for pylint executable path

[corradomonti]

Hello, in some projects I create an ad-hoc environment with conda or venv, while on the most basic ones I use the default Python environment, for simplicity. I think this scenario is not unique.

My proposal would be to allow for the linter-pylint.executable variable to be a comma-separated list of alternatives: the user could specify a per-project pylint path by using the %p syntax, and a fallback e.g. to /usr/local/bin/pylint if the first is not found.

Does this seem reasonable? I can try to put together a PR if there is agreement and interest.