Removed unFormat, unMeta.

The JSON is cleaner without, and we don't use them.
Added unMeta again.
Builder:  plain now produces mempty on empty input.
Monoid instances for Pandoc, Meta.
Switch to left-biased union for mappend of Meta.
Removed extra import.
Walkable instances for Pandoc Pandoc and Meta Meta.
Walk:  Added instance for Walkable a [b].
Generalized docTitle, docDate, docAuthor so they work with MetaBlocks.

This will help with MMD title blocks, where the fields parse as
MetaBlocks [Plain ils].