Add min diff threshold param when making price batch from product query

Hopefully this sets the stage for arbitrary batch params here..

tailbone/templates/themes/better/base.mako

@@ -130,3 +130,12 @@
 <%def name="footer()">
    powered by ${h.link_to("Rattail", url('about'))}
 </%def>
+
+<%def name="wtfield(form, name, **kwargs)">
+  <div class="field-wrapper${' error' if form[name].errors else ''}">
+    <label for="${name}">${form[name].label}</label>
+    <div class="field">
+      ${form[name](**kwargs)}
+    </div>
+  </div>
+</%def>