Rename delimiter_stack to delimiter

The struct actually describes a delimiter. I also find it conceptually
much clearer to think of a doubly linked list of delimiters than a stack.
Simplify push_delimiter

Always modify the subject when pushing a delimiter.
Simplify remove_delimiter
Eliminate delim_count in struct delimiter

Use literal length instead.
Rename member first_inline to inl_text in struct delimiter
Merge pull request #228 from nwellnhof/delimiters

Rename delimiter identifiers