Notifications

[Rich-Harris]

Via. gobblejs/gobble#101. Would be useful to show notifications on build and on error (and also on warning, if transformers gained a this.warn(...) method). Should be behind an option, e.g

gobble --notify    # notify on build or error
gobble -n          # equivalent to above
gobble -g error    # only notify on error

Initial survey suggests node-notifier might be the best option.