Add support for multiple output formats (JSON, zero-separated lines, etc.)

[cipriancraciun]

Add support for outputing in various formats (especially for generate) for the following formats:

• text -- the current plain-text output format;
• text-zero -- \0 terminated lines, meant mainly for tool consumption;
• json -- JSON as a single object / array;
• json-sequence -- a sequence of JSON objects / arrays; (useful for jq handling;)

(Special care must be taken to make sure we don't leak secret material while serializing. See also #8.)