Removes CMARK_OPT_SAFE from options.

Adds CMARK_OPT_UNSAFE, with the opposite meaning.
The new default behavior is to suppress raw HTML and
potentially dangerous links.  The CMARK_OPT_UNSAFE
option has to be set explicitly to prevent this.

--------------------------------------------------------
NOTE: This change will require modifications in
bindings for cmark and in most libraries and programs
that use cmark.
--------------------------------------------------------

Closes #239, #273.

Borrows heavily from @kivikakk's patch in github/cmark-gfm#123.

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.

This didn't cause problems so far because

- all types are 32-bit on 32-bit systems and
- arguments are passed in registers on x86-64.

The wrong types could cause crashes on other platforms, though.

- Fix PATH for api_test, see:
  https://cmake.org/pipermail/cmake/2009-May/029423.html
- DLL is named libcmark.dll under MinGW.

For proper cross-platform paths.