Integrate with Google Analytics

See #350

This commit adds data collection for:

- 'Dart: pub build' command
- 'Dart: Run' command
- 'Install' event

I've left out the 'activated' event because I don't think it's reliable. It seems that ST will trigger the 'plugin_loaded' event under multiple circumstances.

Also, I've added a *dart_enable_telemetry* setting that defaults to *true* and can be used to opt out of data collection for analytics.