Convert built-in commands to argparsing for #250
Convert built-in commands to argparsing for #250

This commit converts the following methods:

- do_eos()
- do_eof()
- do_quit()
- do_shortcuts()
- do_cmdenvironment()
- do__relative_load()
- do_load()
- do_help()

python-cmd2/cmd2,python-cmd2/cmd2