Add analytics plugin

[seaneagan]

• Use github.com/dart-lang/usage to interact with google analytics
• Use the prompt package for an opt-in flow on first usage.
• Add a --[no-]analytics flag which would override the analytics call for that invocation (subsequent invocations too?).
• Send analytics data about which commands and options are being used.
• Don't ask to enable analytics when invoked non-interactively. Possibly disable analytics altogether in this case?