The default is simple, which is backwards compatible.
--format=json will display the list as a json dict

resolves theskumar#405

Allows dumping of all variables in various formats.

Currently defined formats:
   simple:  Each variable is output as <name>=<value> with no
            quoting or escaping. The output is not parseable. This
            is the default format, for backwards compatibility.
   shell:   Each variable is output as <name>=<value>, where
            <value> is quoted/escaped with shell-compatible rules,
            the result may be imported into a shell script with
              eval "$(dotenv list --format=shell)"
   export:  Similar to "shell", but prefixes each line with "export ",
            so that when imported into a shell script, the variables
            are exported.
   json:    The entire set of variables is output as a JSON-serialized
            object