pass file name as an argument to Vlad during initiation

[dmcclory]

We were running SubClassOfVlad.validate() in our own code, instead of using thevladiate` binary. Since that file wasn't in the working directory, we had to do somthing like this:

vlad = SubclassedValidator()
vlad.source = LocalFile(csv_file)
vlad.validate()

Not too bad, but it would be nice to be have an api that's more like this:

vlad = SubclassedValidator(csv_file=csv_file)
vlad.validate()