Document and use longer names for Omd_lexer

Consequences of that in Omd_parser have been taken care of.  Also, the
new function Omd_lexer.split_first allows to remove some generated code.
Trailing spaces are not part of the header.

Fixes https://github.com/pw374/omd/issues/68
Merge pull request #74 from Chris00/headers

Document Omd_lexer & remove headers trailing spaces