logging: flogtool tail/dump should format events

[Brian Warner]

The default mode for both 'flogtool tail' and 'flogtool dump' should be to emit formatted log events, something like:

15:17:01.455 blah blah blah

instead of dumping the raw event dict. A --verbose options should switch it to emit such a dictionary instead. We also need display filters, to remove foolscap.negotiation, or events below a certain level. Also, 'flogtool tail' should have an option to write all events (even undisplayed ones) to a file, so that we can look at them later.

[Brian Warner]

oh, and if there are multiple sources in the logfile (tubids, or different incarnation records), there should be an option to color-code the output according to which program created the message. This suggests that 'flogtool tail FURL1 FURL2...' might be a good idea, to follow multiple programs at once.

[Brian Warner]

• 'tail' and 'dump' now both emit formatted events
• they both have --verbose options
• 'flogtool tail --save-to FILENAME' saves events to a file, readable by 'flogtool dump'

I've moved inline event filters (#53) and color-coded output (#54) to separate tickets.