v0.7.1

v0.7.1: Fix nested markup when pasting inside a Track Changes marker

- Pasting CriticMarkup inside an existing marker no longer nests
  (e.g. {++any++} into an addition). Inner markers are flattened to
  their accept-form before being absorbed, so the marker just grows
  by plain text. Counterpart to the #40 plain-text paste fix.