Check for potential path traversal attack vectors

[LukasKalbertodt]

I thought a bit about this problem, and right now, I don't think reinda itself can be responsible for letting a PTA through. The get method checks either self.setup.assets or self.assets. In both cases, there are only entries that were manually listed in assets!.

However: I should check this more thoroughly and reinda should help to prevent PTA vulnerabilities in user code. For example, one could disallow .. in in the asset paths. Or we can add more docs to relevant base_paths to tell users to beware of PTAs.