Add custom stimulus tag to make it easier to create stimuli in django…

[JulianFeinauer]

… templates.

Type of PR (feature, enhancement, bug fix, etc.)

feature

Description

See https://github.com/jonathan-s/django-sockpuppet/issues/59

Why should this be added

Makes it easier to use.

Checklist

• [ ] Tests are passing
• [ ] Documentation has been added or amended for this feature / update