Replace initialRun with onStart

[gajus]

As reported in #41, the initialRun API might be a bit confusing for first-time users because it triggers onChange with an empty array. A simpler API would be to simply add onStart hook. However, there are trade-offs.

At the moment, initialRun triggers onChange, which if it fails, user could just re-run it the next time a change is detected. Adding onStart means that we need to think of how to handle onStart failing.

Open questions:

• If onStart fails, should it be restarted?
• If onStart fails, should Turbowatch exit with an error?
• Should onChange be blocked until onStart completes?
• How would it work with persistent triggers?, i.e. Would we only trigger onStart and never trigger onChange?

[gajus]

FYI @hyldmo