Make template_name more flexible

[giaume]

Current implementation will look for herald/html/{template_name}.html or herald/text/{template_name}.txt

To make it more flexible, we want to:

1. handle optional slashes in template_name (e.g. my/custom/template/path looking into my/custom/template/{template_name}.html and my/custom/template/{template_name}.txt)
2. based on the type of template_name, if it's a string we keep the current behavior (slightly enhanced with the handling of slashes), and if it's a dict we except something like

        'text': 'my/custom/template/path.txt',
        'html': 'my/custom/template/path.html'

Engineering Todos

• Add tooling for runserver and having a persistent sqlite db

refs worthwhile/django-herald#19