rustdoc-json 0.4.0

## v0.4.0
* Support for specifying `--target`, `--features`, and `--package`
* Make it clearer that `RUSTUP_TOOLCHAIN` and friends has an impact

## v0.3.1
* Don't eat up stdout and stderr

## v0.3.0
* Change `fn build()` to take `BuildOptions`

## v0.2.2
* Make sure `RUSTDOC` and `RUSTC` env vars are cleared before building rustdoc JSON

## v0.2.1
* First public release