0.13.0

Features:

• Add flask_plugin and marshmallow_plugin spec kwargs to allow
  overriding base plugins.
• Backwards-incompatible: Rename plugins spec kwarg into
  extra_plugins.
• Backwards-incompatible: Don't default to OpenAPI version 2.0. The version
  must now be specified, either as OPENAPI_VERSION app parameter or as
  openapi_version spec kwarg.
• Support apispec 1.0.0.
• Backwards-incompatible: Drop support for apispec 0.x.