Handle tables with surprising names

[simonw]

E.g. a table called foo/bar should be dumped to a predictable filename.

I'll probably use the same trick as Datasette for this.

https://github.com/simonw/datasette/blob/f0d32da0a9af87bcb15e34e35424f0c0053be83a/tests/test_utils.py#L202-L215