This is a huge commit since the last one. Here are the major changes:
- Add original source files and the amalgamation generator utilities
- Add dynamic string utilities (arg_dstr) used to generator output
- Add hash table utilities (arg_hashtable) used to store sub-commands
- Add utilities (arg_cmd) used to implement the sub-command mechanism
- Use clang-format to unify the coding style
- Add new unit tests
- Fix warnings under strict compiler settings in VC and GCC
- Add ARG_REPLACE_GETOPT to choose the built-in or system getopt
- Add utilities to generate help messages
- Cleanup the mess after various pull requests