Check for existence of dirName

[seancdavis]

For new projects, it does make sense to rename the project in the way we're doing it here. But if falling back to the default (my-stackbit-site), perhaps only append the timestamp if the directory doesn't already exist. For projects from examples, the same logic could hold true.

Eventually this could be interactive, but even just this check and not having to worry about appending a unique value that isn't necessary would simplify things for the user.