Implement warning if destination directory pre-exists

[Knetic]

Currently, wisk will overwrite anything that it needs to in order to generate a project. That means using wisk on an existing project could completely destroy it, depending on contents. A warning should be implemented which asks the user if they're sure they want to overwrite. A corrosponding -y switch should be used for "quiet" mode.

Message should be;

Target directory exists, use the '-y' switch if you wish to overwrite

[Knetic]

Done as of 6107cd9