Simplify the `Omd` interface.

In particular, less used functions and types have been left in their
own module (to which we refer in the documentation).

Fixes https://github.com/pw374/omd/issues/44
Improve the documentation of `Omd`.
Typo.
Provide a function to build a table of contents.
Remove possible spaces between "#" and the title.

Fixes https://github.com/pw374/omd/issues/46
Fix bug to TOC generation.
Enable special meaning to start=0 for TOC offset.
Use Omd.toc for the program "omd".
Markdown output: no additional empty line at end of code blocks.

Fixes https://github.com/pw374/omd/issues/49
Quick fix for rendering `Ulp` and `Olp` to Markdown.

This is "quick" because one may not want to add so many newlines at
the end of the list, just the required minimum.

Fixes https://github.com/pw374/omd/issues/52
Merge pull request #45 from Chris00/master

Simplify the `Omd` interface.