This commit moves wasm-pack further along the spectrum towards the 1.0
output previously discussed at the last work week. The changes here are:
* Steps which execute near instantaneously no longer print informational
messages by default.
* Long-running steps like downloading chromedriver/geckodriver now only
print if they actually perform a download.
* The "add wasm target" step now prints nothing if the wasm target is
already installed.
* Child process output is no longer captured and is redirected natively
to the terminal, granting colors from rustc/Cargo as well as Cargo's
progress bar during a build.
Alex Crichton