Add a mechanism for extending the Markdown reader and writer

[Witiko]

While the effort to reduce the technical debt in the TeX code (see #96, #119. and #124) has proven useful, the monolithic structure of the Lua parser is worrying and it artificially inflates the cost of implementing future syntax extensions (see #61, #123, and #126). Following the example of the Python-Markdown project, we should attempt to separate a minimal core that implements Gruber's Markdown (with reduced parsers.escapable according to lunamark 0.4.0) and add an extension mechanism that will introduce a more modular structure to the Lua parser.