Please add a --noninteractive option

[zackw]

Currently there's only one place where cargo-llvm-cov might ask the user a question (installing llvm-tools-preview), and this can be suppressed with the CARGO_LLVM_COV_SETUP environment variable, but that variable is specific to that task. ask_to_run also looks at various variables to detect whether it is running in a CI environment, but it is inappropriate to set those variables when not running in CI as they can have other effects.

Please add a command line option (preferably) or an environment variable that means "do not ask any questions ever". The usual name for this option is --noninteractive.

[zackw]

Addendum: Unlike the current behavior of ask_to_run, the effect of --noninteractive should be as if the user said no to all prompts. This is what people expect of programs with a --noninteractive option. (I would argue that this is also what ask_to_run should do if it detects a CI environment, but that's less clear cut.)