Handle user defined tagged literals when parsing application/edn.

Users can define their own tagged literals via data_readers.clj or
associng them to *data-readers*. This map has to be passed into
clojure.edn/read-string or clojure.tools.reader.edn/read-string as the
first argument. Otherwise they won't get recognized and an exception
is thrown. With this change edn/read-string has the same behaviour as
core/read-string minus the security problems.