Test specifications (to be continued)
Add some tests for the headers.
Test paragraphs: "Br" not followed by "NL"
Add more tests for the headers.
Merge pull request #74 from Chris00/headers

Document Omd_lexer & remove headers trailing spaces
Remove trailing white space.
Remove one newline after Br.

Address https://github.com/pw374/omd/issues/67 (not sure it is
completely solved yet).
Simplify the parsing function `spaces`.
Mnemonic name for an argument of `parse`
Restructure the function `spaces`
Take care of blank lines in `spaces`.
Slightly simplify the code to detect a horiz. rule
Make the same decision in `spaces` and the main loop

This is about collapsing newlines into a single one before indented
code.
Use the type aliases in `Omd_parser`
Merge the two decisions about indented code

Fixes https://github.com/pw374/omd/issues/66
Split the `spaces` function according to its usage

The treatment of spaces at the beginning of a line or elsewhere is
different.  Make these two cases clearer.  (Also avoids doing again
tests that were already performed.)
Merge pull request #72 from Chris00/specs

Test specifications (to be continued)
Recognize blank lines with ≤ 3 spaces

Fixes https://github.com/pw374/omd/issues/66
Merge pull request #75 from Chris00/spaces

Improve spaces handling
Move `make_paragraphs` to `Omd_parser`