Add --clean-per-version flag

[taiki-e]

--clean-per-version
    Remove artifacts per Rust version.

    Note that dependencies artifacts will also be removed.

    This flag can only be used together with --version-range flag.

$ cargo hack check --version-range 1.50.. --clean-per-version -v
info: running `cargo clean`
info: running `cargo +1.50 check --manifest-path Cargo.toml` (1/2)
   Compiling ...
    Finished dev [unoptimized + debuginfo] target(s) in 7.70s
info: running `cargo clean`
info: running `cargo +1.51 check --manifest-path Cargo.toml` (2/2)
   Compiling ...
    Finished dev [unoptimized + debuginfo] target(s) in 9.2

Closes #119

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci