API consistency

As we were saying on #10, the API needs some changes:

- [x] `houdini_autolink_issafe` vs `houdini_html_is_tag`  
  **Solution:** Let's pick `is_`.
- [x] `houdini_stack_init` vs `houdini_markdown_new` and `houdini_buffer_new`  
  **Solution:** Let's pick `_new`.