contrib: pick: allow recursive message field formats

contrib: pick: allow recursive message field formats

Previously, the message format was fixed: each part had to be a
certain width and either left or right justified. This allows the user
to specify that two parts can be variable width but that combined they
should be some fixed width. We do this by allowing the user to set as
a "field" a list of the normal result-format form which is formatted
and then itself inserted according to the format string specified.

This means all existing formats work but allows more general things
too. This will be used in the next patch to allow the user to specify
where the tree box graphics are drawn but allow, e.g., the total width
of the tree box graphics and subject to be specified.