Add a mechanism for extending the Markdown reader and writer

[Witiko]

Closes #138.

Tasks

• Prepare reader for extension:
  • Move parsers upvalue and larsers local to reader->parsers.
  • Move syntax local to reader->syntax.
  • Move writer local to reader->writer.
  • Move parse_*(...) locals to reader->parser_functions.parse_*(...).
• Prepare a simple extension for tables:
  • Create a extensions.table(captions) function.
  • Extract table-specific code from writer to extensions.table.extend_writer(writer).
  • Extract table-specific methods from reader to extensions.table.extend_reader(reader).
  • Add reader->apply_extension(extension) method that will call extensions.table.extend_reader(self) and extensions.table.extend_writer(self.writer).
  • In new(options), react to options.pipeTables and options.tableCaptions by calling reader->apply_extension(...).
• Prepare extensions for all remaining options:
  • Options fencedCode and blankBeforeCodeFence.
  • Options citations and citationNbsps.
  • Options contentBlocks and contentBlocksLanguageMap.
  • Option definitionLists.
  • Options jekyllData and expectJekyllData.
  • Options footnotes and inlineFootnotes.
  • Option headerAttributes.

The proposed changes will reduce the code complexity of the Lua parser and perhaps also make it possible for external code to extend Markdown, although such use would be still highly unstable due to the lack of encapsulation in the proposed implementation.