Make JSON prettifying optional

[Kijewski]

This PR adds an optional argument to the |tojson filter, which controls if the serialized JSON data gets prettified or not. The arguments works the same as flask's |tojson filter, which passes the argument to python's json.dumps():

• Omitting the argument, providing a negative integer, or None, then compact JSON data is generated.
• Providing a non-negative integer, then this amount of ASCII spaces is used to indent the data. (Capped to 16 characters.)
• Providing a string, then this string is used as prefix. I attempts are made to ensure that the prefix actually consists of whitespaces, because chances are, that if you provide e.g. &nsbp;, then you are doing it intentionally.

This is a breaking change, because it changes the default behavior to not prettify the data. This is done intentionally, because this is how it works in flask.

[GuillaumeGomez]

Looks much simpler now, thanks! Please feel free to merge once CI pass.