Inserting source and other fields

[stuartpb]

As required for #3, #4, (some of) #7, and (probably) #9.

Plugins providing logs need to start inserting a field on the line after the timestamp to distinguish the message's source (probably using sed), matching the format of logs from Heroku.

Heroku just has the "source" that comes after the timestamp; I'm not sure if syslog-style importance would be a user convention or what, but it seems like source is the only field that is necessary.

Maybe the hook name should be changed so as to not lock app-logs into a shortsighted format (allowing new names for new formats in the future)? eg. timestamped-app-logs and timestamped-sourced-app-logs.

[stuartpb]

Per plushu/plushu-app-docker-logs#1, one sensible piece of data to track in sources is the source/level of a log message (something I'm not sure even Heroku tracks atm). Decoration conventions we could follow / take inspiration from:

• syslog
• Debian's annotate-output