Add json flag

[jopemachine]

Fixes #50

I replaced the logUpdate API in the previous PR with ink's API.
And to avoid showing upload-attributes from the json result when upload is false, added these lines.

I'd appreciate letting me know if there are other tasks to do :)

[sindresorhus]

The --json output cannot be interactive. It should be possible to redirect the output.

[sindresorhus]

It should not include the isDone property.

uploadSpeed should also not be included when not using --upload.

[sindresorhus]

I don't think we should include "downloadUnit": "Mbps". The downloadSpeed and uploadSpeed should always be converted to Mbps if the returned value is not in Mbps.

[jopemachine]

Thanks for your detailed review :)
I just updated PR.

[sindresorhus]

I think you missed: #74 (comment)

[jopemachine]

I think you missed: #74 (comment)

I'm sorry for missing your point.
I just fixed the PR assuming the convert should be applied when using json option.
If I got wrong, I'd appreciate your letting me know.

[sindresorhus]

Thanks :)