Allow Reader in addition to String and java.io.File

[daveliepmann]

Per read-csv's docstring, parameter uri should be able to be a Reader such as from clojure.java.io/reader. This PR makes that possible. My use case is something like:

(with-open [rdr (io/reader (io/resource "iris.csv"))]
  (doall (csv/read-csv rdr {:fields [:sepal-length :sepal-width
                                     :petal-length :petal-width
                                     :species]})))

Ideally meta-csv would accept java.net.URL so that the resource could be read directly, but that isn't currently listed as an acceptable parameter. But adding (instance? java.net.URL uri) to this or would actually solve my use case more directly.

(I did not look into InputStreams but there may be a similar issue there.)