Don't require "." prefix for dot commands

[jamesls]

Additional discussion here: https://github.com/awslabs/aws-shell/issues/76

The idea is that similar to how the % isn't required in ipython (if automagic is enabled), we don't have to require the . prefix. Additional notes:

• These commands would have the lowest precedence. Conceptually we'd check to ensure there's no CLI command with the provided name, and then fall back to the dot command handler.
• If there's ever a CLI command with the same name as a dot command, you'd always have to specify the . prefix for the shadowed dot command.
• ipython has a config option to enable/disable this behavior. Need to discuss if we want this configurable or just always enabled.

[jleaders]

I second this, especially since typing exit quit or close or pressing F10 or CTRL-C on Ubuntu will NOT close the terminal. Very frustrating. It's almost as hard to exit as VIM. In fact I still have a copy of VIM running from 3 years ago I don't know how to close.