Brainstorm potential Fire CLI configurations

I'm starting a list of things people might want to configure in their Fire CLIs

- Set a function to always show --help, rather than trying to display the output. (https://github.com/google/python-fire/issues/47)
- Prevent CLI access to private members (https://github.com/google/python-fire/issues/47)
- Prevent CLI access to specific members (https://github.com/google/python-fire/issues/47)
- Configure how a particular type is serialized
- Set a parse function for a particular input argument ([L32](https://github.com/google/python-fire/blob/master/fire/decorators.py#L32))
- Cache a particular input to a fn
- Cache a function's output
- Set a custom help string
- Make a member available by a different name.
- Make an argument available by a different name.
- Enable single-letter options.
- Allow passing arguments via stdin (what would be the syntax?)
- Whether to use environment variables as arguments (https://github.com/google/python-fire/issues/94)
- What prefix to strip from environment variables (https://github.com/google/python-fire/issues/94)
- Enabling post mortem debugging (https://github.com/google/python-fire/pull/71)
- Don't parse arguments to a function (pass them all as strings) (would enable https://github.com/google/python-fire/issues/29)
- How verbose should Fire be? (https://github.com/google/python-fire/issues/77)
- Freezing completion autogeneration after #32 is implemented (#32)

**By no means do we plan on supporting all of these**; just good to keep a list.

The primary reason for making this list is so that as we start adding support for these, we can do so in a way that's forward looking. We want any configs that we introduce to look and feel consistent with each other, and not to later need to be renamed or removed for consistency with other configs added at a later date.