Enhancements for standard library's cmd module.
Drop-in replacement adds several features for command-prompt tools:
* Searchable command history (commands: "hi", "li", "run")
* Load commands from file, save to file, edit commands in file
* Multi-line commands
* Case-insensitive commands
* Special-character shortcut commands (beyond cmd's "@" and "!")
* Settable environment parameters
* Parsing commands with flags
* > (filename), >> (filename) redirect output to file
* < (filename) gets input from file
* bare >, >>, < redirect to/from paste buffer
* accepts abbreviated commands when unambiguous
* `py` enters interactive Python console
* test apps against sample session transcript (see example/example.py)
Useable without modification anywhere cmd is used; simply import cmd2.Cmd
in place of cmd.Cmd
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