Quotes and brackets as multiparagraph divisions

This commit introduces a new concept of multiparagraph divisions (where
  the lines within are re·analysed after the prefix is removed) and
  transitions block quotations to it.
I resisted this for a long while because it¦s less “simple”, but the
  precedent of preformatted text and the old quotation mechanism
  (which was kind of messy) was already conceptually close.
And footer support really requires this.