Adds ability to force json or xml parsing during templating

[gislikonrad]

When requesting HTTP endpoints not under your control, you could come across an endpoint that doesn't respond with the correct content-type header value. For example getting an xml body, but the header says it's text/plain. You could also get a json body and there is no content-type header.

With this PR, I add a way to force parsing as json or xml when templating.

For JSON it would be like this: {{previous_request_name.response.body.asJson.$.data.id}}
For XML it would be like this: {{previous_request_name.response.body.asXml.//data/@id}}

[JerryJian]

I need this feature, because my response header is text/html, but the content is json format. Looking forward to the merger this PR, thanks. @Huachao

[Huachao]

@gislikonrad @JerryJian merged, thanks