Make lobos.migration/ns-file support options in libspecs

Make ns-file able to support use options when resolving namespace file path.

Example:

If `foo.bar` and `foo.baz` both have a var named `qux`:

    (doseq [mns ['[foo.bar :exclude [qux]] '[foo.baz :exclude [qux]]]
      (binding [*migrations-namespace* mns]
        (do-something)))

or simply:

    (binding [*migrations-namespace* '[foo.bar :exclude [qux]]]
      (do-something))