Stringify objects

[slayer]

Currently objects displayed as [object Object]

for example, for event {message: "blabla", meta: {ip: "1.2.3.4", user_id: 123}} and format "{{message}} {{meta}}"
Result will be "blabla [object Object]"

This PR adds helper to display objects as JSON:
"{{message}} {{json meta}}"
Result will be "blabla {ip: "1.2.3.4", user_id: 123}"

[sivasamyk]

Thanks for the PR! Can't you directly map the json fields in handlebar like {{{meta.ip}}}? Are the fields in meta dynamic? If so, won't it be useful if we prettify the json using JSON.stringify(context, null, 2);?