Also introduces testing of our CLI's output via `assert_cmd`. Expect some follow
ups to get more of our testing infrastructure using this incredible crate!
Fixes#511
This commit replaces the `slog` family of crates used by `wasm-pack`
with the `log` crate plus `env_logger`. This also means that by default
`wasm-pack` also won't create a `wasm-pack.log` file in the current
directory. Enabling logging will now be done through
`RUST_LOG=wasm_pack` instead of `-v` flags.
Closes#425
Pauan