API change: This adds a new exported function in cmark.h.

Closes #330.

Closes #188.

@nwellnhof - could you have a look and let me know if you
think this is a bad idea or could be improved?

This reverts commit 9e643720ec903f3b448bd2589a0c02c2514805ae.

This reverts commit 4fbe344df43ed7f60a3d3a53981088334cb709fc.

* Improve strbuf guarantees

Introduce BUFSIZE_MAX macro and make sure that the strbuf implementation
can handle strings up to this size.

* Abort early if document size exceeds internal limit

* Change types for source map offsets

Switch to size_t for the public API, making the public headers
C89-compatible again.

Switch to bufsize_t internally, reducing memory usage and improving
performance on 32-bit platforms.

* Make parser return NULL on internal index overflow

Make S_parser_feed set an error and ignore subsequent chunks if the
total input document size exceeds an internal limit. Make
cmark_parser_finish return NULL if an error was encountered. Add
public API functions to retrieve error code and error message.

strbuf overflow in renderers and OOM in parser or renderers still
cause an abort.

* open_new_blocks: always create child before advancing offset

* Source map

* Extent's typology

* In-depth python bindings