Add option for cleaning up output folder in CLI

[mimarz]

We need to be able to clean up old tokens and generated theme css files to avoid bugs and conflicts.

If users want to rename color tokens and create new tokens, we get dangling design tokens with no connection. This will also be a use-case if they have used tokens build for css files. This can cause errors with duplicate definitions on selectors like :root for variables that no longer exist.

Suggestion:

• By default, we delete the content of output folder, if files exist there, prompt user with warning for y/n about files being deleted to make way for new ones
• Users can choose to suppress the warning by adding an argument, someting like --force or --unsafe