Improve output of tree integrity check
Merge pull request #206 from nwellnhof/improve_node_check

Improve output of tree integrity check
html: Removed union from RenderStack.

It doesn't make sense to have a union here, and this simplifies
the code.
Updated benchmarks.
Fix to #205 that doesn't require raw HTML.
Initialize all fields in node when creating inlines.
Added remarkable.js benchmark.
Make render_html support nodes with no children

For empty inline nodes like EMPH, the parser always creates a child
containing an empty string. Using the tree manipulation API, nodes with
no children can be created. Adjust render_html to cope.
Add node constructor and accessors to the public API

The approach I'm taking is to copy inline literals internally to
NULL-terminated C strings if requested by an accessor. This allows to
return a 'const char *' that doesn't have to be freed by the caller.
Start with tests for the C API

The C API tests can be run individually via

    build/api_test/api_test

Or together with the spec tests via

    cmake --build build --target test
Added a few more benchmarks.
Boldface commonmark.js too in benchmarks.
Try to support older CMake versions
Add header file to CMake api_test sources
Set compiler flags for api_test
Fix api_test warnings
Removed remarkable.js benchmark.
Merge branch 'api_tests' of https://github.com/nwellnhof/CommonMark into nwellnhof-api_tests
Add api_test directory to archive.
make test: Use cmake to run tests.  So we get api tests too.