use --vanilla option to speed up hooks and decouple from .Rprofile

[lorenzwalthert]

Currently can't really think of a use case where .Rprofile or related is useful. All important options should be exposed as args:.

Edit: Not so easy because we'd have to use system as the language instead of script and change the entrypoint to

Rscript --vanilla inst/bin/style-files

But then the style-files file is not found anymore (obviously). So I think this does not work. We can also not provide --no-vanilla easily in the shebang apparently to make it work for all operating systems.

[lorenzwalthert]

Added a note in 4bcad27 and opened pre-commit/pre-commit#1426 so peole could use this env variable in their .Rprofile to skip startup stuff while pre-commit hooks are running.