Add '@=' separator to pass JSON objects via CLI

[agoconcept]

The current '=' separator enables to add key-value pairs as strings in a JSON object to the HTTP body. But there's no way to provide more complex JSON objects

This commit introduces a new '@=' that adds a key-value pair where the right value is evaluated to a JSON object, allowing more complex structures in the HTTP body.

Change-Id: I126a6c8529a6dbf9108b904b2b4d93ef60309113

[agoconcept]

Is there any additional change needed to accept the pull request? In case anything else is needed please don't hesitate to ask for it

Cheers!