Add 'squeeze' argument to Parser init, capture_body, to strip any enclosing length-one lists

[bskinn]

It's common with Parsers as-is to have to do a bunch of ...[0][0] indexing to drill down to the data you want. Adding an option that automatically does ...[0] indexing until a len(l) > 1 list (or a non-list singleton) is obtained would be an attractive way to ease the syntax. Must cope with a final length-zero list...perhaps reduce to None here.

Figure an argument squeeze to capture_body makes sense. Probably not useful on capture_struct. Might also make sense to have squeeze be an argument to Parser initialization, that sets a per-Parser default. capture_body default can be a sentinel, indicating 'defer to Parser setting'.

Figure default should be False, since the squeezing represents a loss of generality/consistency in the output in the event that more or less content is matched than expected.