Provide a validator for an existing file

[blueyed]

If you have an attribute for a file name, you might want to validate that the file actually exist.

It that something to be useful with the default validators? (http://www.attrs.org/en/stable/api.html?highlight=validat#attr.validate)

[hynek]

I guess? I think I'd merge a PR that does that.

We should orient ourselves on what click is doing with Path.