Renamed identifiers in public API:

cmark_doc_parser => cmark_parser
cmark_new_doc_parser => cmark_parser_new
cmark_free_doc_parser => cmark_parser_free
cmark_finish => cmark_parser_finish
cmark_process_line => cmark_parser_process_line
cmark_node_destroy => cmark_node_free

Closes #223.