Release v0.11.0

This release contains several API changes. These are mostly compatible with
observed existing use, but may require some updates.

Incompatible changes:
  - Change the signature of start functions to no longer return *Group.
  - Remove the ErrorFunc type (no longer used).
  - Change the argument type of New and OnError to any.
  - Change the return type of Trigger and Listen to any.

Other changes:
  - Add OnError, allowing an error filter to be set/changed.
  - Add Throttle type, allowing multiple groups to share a limit.
  - Add Gatherer type, to replace Collector.

Deprecations:
  - Trigger and Listen functions. Their arguments can be passed directly to New
    and OnError instead.
  - Collector type and Collect constructor. Use Gather instead.