Fixes #26 Textarea now respects rows attribute (#859)

* textarea now respects rows attribute

* use the :not() selector

* better sass selectors

sass/elements/form.sass

@@ -70,12 +70,15 @@ $input-radius:              $radius !default
 
 .textarea
   display: block
-  max-height: 600px
   max-width: 100%
-  min-height: 120px
   min-width: 100%
   padding: 0.625em
   resize: vertical
+  &:not([rows])
+    max-height: 600px
+    min-height: 120px
+  &[rows]
+    height: unset
 
 .checkbox,
 .radio