notably, test normal + single + generated + Lua 5.3.5 only once,
then only run the runtime_test and compile_test for normal for all other permutations to help increase text matrix throughput
Added the ability to extend all usertypes at runtime. The performance implications need to be examined closely.
variadic_args documentation was updated with the desired example demonstrating proper usage
usertype examples were updated demonstrating Lua runtime and C++ runtime updating of a usertype table
SOL_SAFE_FUNCTIONS is now part of the definitions and defined (thanks @eliasdaler)
