feat: toggling returns visible value

[osamuaoki]

Here is my first try with code changes and document updates in 2 commits.

In order to be consistent, I added return value for all toggle, show, and hide; but not for peek.

Let me know if you want to limit changes to toggle only.

closes #72

[tris203]

I think this should just be scoped to the toggle command

The value of the show and hide values is implied

[osamuaoki]

Done and force-pushed

[tris203]

Thanks!