Initial support for json output

[jLantxa]

Implement the json reporter and support the most important commands.

• init
• log
• snapshot
• forget

cmd_clean is not so important for now and cmd_restore will be redesigned soon.

[jLantxa]

• The snapshot commands needs some refactoring. To separate reporting from stats collection, we need a status struct and a reporter. The CLI and JsonSnapshot reporter must be separate and we choose the reporter in cmd_snapshot.
• Restore might need the same refactor, but since we're redesigning it, there's no need yet.
• cmd_clean can also wait too.