v3.1.0

This release has huge differences from 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