Add nodiscard support, enhanced warnings

This release is the first substantial release in some time. Changes:

Support for nodiscard
Additional warnings enabled (and fixed) for -Wextra on clang
Removed outdated and inaccurate help in the header, refer to helpfile.md for help topics
Cleaned up implementation of exception handler options to be readable and maintainable. Prevents inclusion of operating system specific headers unless you specifically opt into something that requires them.
Completed safe_math.h, now have check_ versions of add, subtract, multiply, and divide for built-in types (int, long, etc)