Add noop str config parser for documentation.

This allows str config params to be specified in plugins so their param
delcarations can be comprehensive.

r2/r2/lib/configparse.py

@@ -27,6 +27,10 @@ import re
 class ConfigValue(object):
     _bool_map = dict(true=True, false=False)
 
+    @staticmethod
+    def str(v, key=None, data=None):
+        return str(v)
+
     @staticmethod
     def int(v, key=None, data=None):
         return int(v)