fix: stack overflow caused by non-references

A reference-looking element was re-parsed
again and again at the backend-level, instead
of correctly falling back to some "sober" text.
fix: windows-style line break

fix https://github.com/ocaml/omd/issues/119

`\r\n` elimination part rewritten because it generated a somewhat wrong stream of lexemes.

modified:   omd_lexer.ml