Replace value_json in templates with filters

[mcdeck]

When template support got implemented to replace the json parsing I originally created for the mqtt component I did like the idea of the template approach. However, I disliked the json support because it basically tries to parse every message as json. Supporting other message formats would require additional value_XXX keys in the templates and that seems quite inflexible.

With templates however we can use filters and I figured I'd give that a try to see if this wouldn't be a nicer implementation of json support in messages. I have now implemented two filters json and jsonpath. The idea is to create a set of filters ready for use but also find a way to allow for custom filters created by users (e.g. in their configuration directories maybe?)
With the two filters in this PR you can do something like this in your config:

value_template: "{{ (value|json).val }}"
value_template: "{{ value|jsonpath('val') }}"

This pull request is meant to illustrate the idea - the implementation is for sure far from perfect and is missing tests, but I first wanted to collect feedback to see if investing more time into this made sense or not.

[balloob]

Adding a way to add custom filters sounds like a good plan. It will allow new components to register filters too. The annotation should probably just live in the template package, no need for an extra file.

The reason I went for the value_json approach instead of value | json is simplicity. Imagine we're trying to extract the value 'hello' from [{"hello": "world"}]:

{{ (value | json).0.hello }}
{{ value_json.0.hello }}

I wouldn't want to add support for other formats but JSON is common enough.

I would not want to add the jsonpath filter. There should be only 1 way to do something and using jsonpath offers the same as a template.

[mcdeck]

I will create a new PR once I have a working solution