Removed cmark_ prefix on chunk and strbuf.

This isn't needed any more since we don't expose these in the API.
Makefile: Removed dependency of leakcheck on PROG.
Remove CMARK_NODE_REFERENCE_DEF from API.

Modified finalize in blocks.c to return parent of finalized
block, so we can handle the case of reference definitions,
when we simply remove the finalized block.
API improvements: cmark_event_type parameter for walk handlers.

Added cmark_event_type enum, which is used as the second
parameter of the handler passed to cmark_walk.

See #224.
cmark:  Replaced `--man` and `--ast` with generic `--to` option.