Make use of refresh_interval parameter for prompt-toolkit sessions

[tleonhardt]

In prompt-toolkit, you can control session and UI refresh rates by configuring the refresh_interval in your Application / Session constructor, which invalidates and redraws the UI automatically.

We should add this as a parameter to the cmd2.Cmd.__init__ method with a default value. The default value should probably match whatever the default value from prompt-toolkit is.

[tleonhardt]

@kmvanbrunt What remains to be done for this? I think the change has been made and we just need to update the CHANGELOG.md and documentation?

[kmvanbrunt]

@kmvanbrunt What remains to be done for this? I think the change has been made and we just need to update the CHANGELOG.md and documentation?

I've made these changes on the #1700 PR.