Add a Jinja filter to parse json strings

[loicalbertin]

Hi,

It would be handy to have access to a filter that could deserialize a JSON strings into a Python object in templates.
I typically have in mind something similar to the from_json filter in Ansible.

I can quickly create a PR to handle this particular use-case but maybe you want to think about more generic way to add new filters, WDYT?

[loicalbertin]

I opened a PR #2 to showcase what I expect.
It could be a starting point for a discussion.

Cheers!