Add jinja fromjson custom filter

[delenamalan]

Add a custom Jinja2 filter, fromjson, to convert a JSON string to JSON - which can then be used by Jinja.

This is useful for formatting notifications based on JSON API responses.

If the API route you're watching returns a JSON list, for example, you could format a notification like this:

<ul>
{% for item in current_snapshot %}
  <li>{{ item.title}}: {{ item.price }}</li>
{% endfor %}
</ul>

[dgtlmoon]

Can you give some (any) information here? whats your use case exactly?

any examples??

[delenamalan]

@dgtlmoon sure, I added a description and example to the PR description now.

The PR is still in draft because I'd still like to add a test and update the docs.