Custom EDN tags

Hi Nils,

I just noticed that custom tags defined in the data_readers.clj
file of an application are not handled by ring-middleware-format.

When a Clojure applications starts, tags defined in
data_readers.clj are loaded into the *data-readers* dynamic var.
However, the default behaviour of edn/read-string is to care only
about the tags in default-data-readers.

Passing {:readers *data-readers*} as the first argument to
edn/read-string solves this.

Would you like to merge this into master?

Thanks, Roman.