Ability to modify default Usage string about cyclopts

[BrianPugh]

Ah yes, I forgot about usage.

I'll do this two-fold:

1. Improve the current usage to be more consistent with other libraries (the current was a bit of a lazy place holder).

2. Allow manual override via a supplied string usage="My Usage".

3. Allow disabling it via supplying an empty string.

[Skyler-Altol]

@BrianPugh would it not be more consistent with the general patterns of python (and this project) to use None for the disabling component? Just a thought :)

Also, great work with this project, I was a heavy user of Typer before I came across this in one of the issues as a resolution to an issue that Typer has refused to fix for years. I'll be raising some ideas in here myself soon ❤️

[BrianPugh]

In Cyclopts, None always resolves to "default behavior", which is also a common python pattern. Typically, the default behavior is to "derive data from some source." Frequently, the defaults can be overridden with a user-provided string, so an Optional[str] type makes sense. Finally, what if the user want's to disable a feature? A natural extension of this is the manual override of an empty string.

When a user-provided str value doesn't make sense (e.g. show_env_var), a bool is a natural choice.

[BrianPugh]

closing with the release of v2. We can reopen this issue, or open a new one, should the need arise.

I'll work on improving the default usage string to be at parity with other tools like argparse/click/typer in the near future.