Reverting "Remove pst-str; use with-out-str instead."

Need to wait for a full version bump to do this.

This reverts commit f28bb79.

@@ -116,6 +116,13 @@
   [& [e]]
   (pst-on *out* false (or e *e)))
 
+(defn pst-str
+  "Like pst, but returns a string instead of printing that string to *out*"
+  [& [e]]
+  (let [sw (java.io.StringWriter.)]
+    (pst-on sw false (or e *e))
+    (str sw)))
+
 (defn pst+
   "Like pst, but with ANSI terminal color coding."
   [& [e]]