Use JSON format in keyword check#693

Allow the keyword monitor to also work with JSON.
This way it is easy to check if an API returned a certain key/value.

This could also be used for product stock monitor since alot of webshops return the stock via JSON but handle it via JS and thus make it inpossible to monitor stock.

Hi there, currently you can check whether the string is presented in the JSON reponse body (key or values) as it was a text. I think you can cover like 95% of cases with current solution. So just to be sure, you want to work with the JSON repsonse like it was a JSON, not a text, right?