Include missing key value in assert message

[rschlaikjer]

This adds the macro RAPIDJSON_ASSERT_MSG, which by default proxies back to the base RAPIDJSON_ASSERT.

With the cassert backend it only works for literal messages, since cassert relies on macro stringification, but by overriding it to use BOOST_ASSERT_MSG it's possible to use dynamic messages.

To take advantage of the new behaviour, callers should add

add_definitions("-DRAPIDJSON_ASSERT_MSG=BOOST_ASSERT_MSG")