Add clean subcommand

[taiki-e]

$ cargo llvm-cov clean --help
cargo-llvm-cov-clean 

Remove artifacts that cargo-llvm-cov has generated in the past

USAGE:
    cargo llvm-cov clean

OPTIONS:
        --manifest-path <PATH>    Path to Cargo.toml
        --color <WHEN>            Coloring [possible values: auto, always, never]
    -h, --help                    Print help information

This also improves heuristics around artifact cleanup.

Closes #69

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci