Added useful 'string' alias to attrs_extra

pillar/attrs_extra.py

@@ -1,9 +1,12 @@
 """Extra functionality for attrs."""
 
+import functools
 import logging
 
 import attr
 
+string = functools.partial(attr.ib, validator=attr.validators.instance_of(str))
+
 
 def log(name):
     """Returns a logger attr.ib