Feature: allow to limit how many instances of a plugin can be created in a placeholder

[vdboor]

As suggested at django-fluent/django-fluent-pages#9, it's valuable to limit how many instances of a plugin can be created in a placeholder. Django CMS also has this feature (see http://docs.django-cms.org/en/2.2/getting_started/configuration.html#cms-placeholder-conf)

This feature is related to #3

[vdboor]

As of 0.9, there is a FLUENT_CONTENTS_PLACEHOLDER_CONFIG setting.
This feature needs to be included there.