added output modes to colorize json output

[prune998]

This PR is about adding two new output modes to Stern:

• extjson is an extended json mode where the whole output is embeded in json and the namespace/pod/container values are color-coded as in the default mode.
• ppextjson is the same as the above with pretty-print, so each field is on its own line.

Both of those modes can be piped into jq if you want full pretty-print, but you'll lose the original color-coding.

I think those options are usefull when you're dealing with Json structured logs. It helps visializing the informations without being too intrusive.
It's better than using the raw output or raw | jq '.' as pretty-printing the whole log line can quickly fill the console if you have lots of logs.

It's not a revolution but I guess it does not hurt to add this feature :)

ex:

./stern_cli nodepool-labeler-controller-manager --tail 2  --all-namespaces --output extjson -c manager

./stern_cli nodepool-labeler-controller-manager --tail 2  --all-namespaces --output extjson -c manager | jq '.'

./stern_cli nodepool-labeler-controller-manager --tail 2  --all-namespaces --output ppextjson -c manager

[rkmathi]

@prune998
Thank you for your implementation!
I think this feature is very useful.

@superbrothers @floryut
What do you think?

[floryut]

@prune998 Thank you for your implementation! I think this feature is very useful.

@superbrothers @floryut What do you think?

Great! Thank you for your contrib @prune998
please review @superbrothers comments 👍

[prune998]

All done ! Please merge if you think it's ready !

Many thanks all for supporting Stern !