Add a run-chromium script.

Builds and runs the extension in a fresh Chromium profile.

cc @cooperq for review.
Going to reccomend that we use the following syntax instead of `#!/bin/bash -ex`:
    set -o errexit
    set -o xtrace
    set -o nounbound
or `set -uex`
because with the former the options will be ignored if the script is run via bash like so: `bash ./run-chromium.sh`