added variadic_results, to return a variable number of arguments to Lua
added variadic_results and as_results
added improved function examples (for multiple results and split overloading out)
added tests for variadics
added tests for C++17 utilities
added a forwarding header
added a specific `unsafe_function` header
added and improved documetation pages
sol::this_state is a transparent argument that gets the current state at any position in any callback
sol::variadic_args allows a person to get something that can reference the "rest of the arguments", though it doesn't enforce that it has to be the last argument
Closes#57Closes#59Closes#60