This is best practice for consistency of appearance between built-in and custom commands.

# Conflicts:
#	docs/features/settings.rst

- In docs/api/cmd.rst, the `cmd2.Cmd` class was defined as `cmd2.cmd2.Cmd`, which required the extraneous `cmd2` every time we referenced it. This extra `cmd2` is no longer required.
- always refer to a bare cmd2 using ``cmd2`` per our documentation conventions

Also:
- Added a "linkcheck" task to the invoke tasks
_ Use pseudoterminals for invoke tasks with colored output so that we can see the color when using invoke

For #765

For #765