Add cmark_node_insert_after to cmark.h

Fix copy/paste error.
Add tests for insert_{before|after} functions
Rename {get|set}_content to {get|set}_string_content
Accessors for string content of code and HTML blocks
Accessors for header level
Accessors for list data

Only 'list_type', 'start', and 'tight' should be relevant for rendering.
Accessors for other list data could be added for completeness but they
don't seem very useful.
Accessors for fence info

Only fence info should be relevant for rendering. Accessors for other
fenced code data could be added for completeness but they don't seem
very useful.
Accessors for link titles
Accessors for start_line, start_column, end_line

These are read-only as they're only metadata returned by the parser.
Fix {get|set}_string_content
Fix api_test summary
Accessor tests
Input validation for setters
Merge pull request #208 from nwellnhof/more_accessors_and_tests

More accessors and tests