`capture_exceptions=False` by default on new contexts if posthog is not capturing exceptions

* If you aren't using posthog for exceptions, that preference is not respected when using the new contexts API. You have to `with posthog.new_context(capture_exceptions=False):` on each and every context.
* Also, there's not a way to easily set the `default_client` which is used by the posthog contexts
* `default_client` is not documented in the python documentation