Mutations and error handling

This release adds support for:

• Mutations - use the ::mutate event. Thanks to @r0man

This release changes:

• Callback events now receive the entire payload map, consisting of :data and :error keys and possibly more. This is a breaking change and any existing callback events will need to destructure the payload map to obtain the :data value which was passed in previously. See the README or tests for examples. This change was added in order to provide better support for error handling, as it now gives you access to the :error key, if present, to use in this workflow. Thanks again to @r0man