Add command `smt view` to show a single record on the command line.

[apdavison]

This should take a single argument - the label of the record to be viewed (or the special value 'last').
With no options, this should show the entire record. There should be an option to view all the components of a record, e.g.

smt view --script <label>

would show just the main script for a record (also see #342)