Add alternative signatures for Model#start and Model#on that don't use var-args

[ericyhwang]

The var-args in the middle of parameter lists for Model#start and Model#on make it difficult to write TypeScript type definitions for them.

These backwards-compatible changes add alternative signatures that don't use var-args.

1. Model#start now supports a new array format for input paths:

model.start(outPath, inPath1, inPath2, ..., fn);  // Old
model.start(outPath, [inPath1, inPath2, ...], fn); // New

2. Model#on has a new {useEventObjects: boolean} option. If true, it calls the listener with an Event object and the wildcard-captured segments as an array:

// Old
model.on('change', 'foo.**', (captures..., value, previous, passed) => {
});
// New
model.on('change', 'foo.**', {useEventObjects: true}, (changeEvent, captures) => {
  const {type, value, previous, passed} = changeEvent;
});

Once this is published, I'll update derby-site with documentation on the new signatures and event objects.

[nateps]

See comments for changes

[ericyhwang]

Made changes based on additional discussion in person:

1. Added a {useEventObjects: true} option to Model#on, which causes the callback to be invoked as cb(event, captures) instead of the now-legacy var-args cb(captures..., [eventType], eventArgs..., passed). This is more explicit than argument-detection, and it enables the use-case you brought up of passing in a scoped model.
2. Split old code in modelEventListener out into a separate modelEventListenerLegacy function.
3. Because we no longer need argument-detection, I've reverted the change to take the pattern as an array of path segments. Even though we still want to move that direction longer term, it's weird to have Model#on be the only thing in the public API that accepts path segments.

[nateps]

Some minor additional comments. LGTM with these resolved