Parser Update:

- revisions to header class and section handling
  header pattern simplified
  header class now writes section open and close instructions
  section call writer removed
  check for and update meta['first_header'] if required (see below)

- meta instruction added
  written by _finalize to front of instruction stack
  holds first header information

  plugins can now write first header information, or other page meta information
  to this data structure.

- get_first_heading updated to make use of meta['first_header']

- plugin instruction structure changed to include lexer state in the data portion
  of the instruction.
  OLD INSTRUCTION:  array('plugin', array(<plugin name>,<plugin_data>,$pos), $pos)
  NEW INSTRUCTION:  array('plugin', array(<plugin name>,<plugin_data>,$state), $pos)

- block handler/call writer update to better handle plugin PTypes 'block' & 'stack'.
  Lexer states are mapped as follows:
    DOKU_LEXER_ENTER, DOKU_LEXER_SPECIAL     : block_open, stack_open
    DOKU_LEXER_EXIT, DOKU_LEXER_SPECIAL      : block_close, stack_close
    DOKU_LEXER_MATCHED, DOKU_LEXER_UNMATCHED : plugin must handle <p>

  Plugin writers can now use these PTypes as intended !

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