feat(log): Add json console log formatter

[rvadim]

If set log config to "name: json_console", messages to stdout will be in json format each message is one object. It is conveniently for sending logs to elasticsearch storage.

Tested with master branch of vulcand. Generating messages like

{"appname":"vulcand","filename":"supervisor.go","function":"github.com/vulcand/vulcand/supervisor.initProxy","linenumber":356,"message":"No frontends found","pid":25521,"severity":2,"timestamp":"Nov 30 03:59:00.130"}