Support dumps in json format

[gavv]

Currently, we support dumps in csv format. It would be nice to also support json.

Json dumps can be useful when dumps and measurement reports are combined in the same file (or stdout). This way, all output will be in json, and it will be easier to parse such combined output.

Steps:

• add --dump-format option (similar to --report-format), with possible values "csv" and "json" and default value "csv"

• rename CsvDumper to something like TextDumper

• teach TextDumper to produce json or csv depending on mode

[pravintargaryen]

@gavv please assign this issue to me

[gavv]

Sure, thanks!