Refine `unit list` command

[shonfeder]

tracer list

will list all logical units in the current context.

See #6

[shonfeder]

The core functionality was implemented in #58, but the end result is a bit noisy.

So I'm refitting this issue to track the need to refine the presentation of the logical units, and add a sub command to show the full info about a unit given its tag. The behavior should be

• [x] unit list only shows a clean, helpful excerpt of the tag, and its content
• [x] unit list supports the --fmt command
• [x] unit show TAG shows the full info about a tag in a nice, human readable table
• [x] unit show TAG supports the --fmt command