Add json / logfmt format options

[RichardWright]

Is your feature request related to a problem? Please describe.
This is a handy lib, ideally it would plug into a standard display format as used by grafana (formats in here - https://grafana.com/docs/loki/latest/logql/#Metric-Queries)

Describe the solution you'd like
Return Json or logfmt

Describe alternatives you've considered
Writing long hand regex, but that seems pretty brittle.

[hg-pyun]

Good Idea. I'm using a Morgan now, so I'll check if it can be easily mashup.

[RichardWright]

I've written a change for this, but I need advice as it's definetly not ok as it stands.