Don't require "." prefix for dot commands

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.