Support context on a per-event basis

[greyvugrin]

The batch endpoint allows individual context, per https://segment.com/docs/connections/sources/catalog/libraries/server/http-api/#batch

context Object, optional The same as Context for other calls, but it will be merged with any context inside each of the items in the batch.

This updates the track and identify methods to take in an optional context array, which carries through to the final API call.

It's useful to be able to pass in page context or anything that may change per event.

[slashequip]

Thanks for this @greyvugrin - and sorry it's taken me a long time to get to it. I'll review this MR and see if we can get it in to the package :)