units: support mm

Closes #161

lib/squib/api/units.rb

@@ -13,5 +13,10 @@ module Squib
       @dpi * Squib::INCHES_IN_CM * n.to_f
     end
 
+    # DSL method. See http://squib.readthedocs.io
+    def mm(n)
+      @dpi * Squib::INCHES_IN_CM * n.to_f / 10.0
+    end
+
   end
 end

lib/squib/args/unit_conversion.rb

@@ -15,6 +15,8 @@ module Squib
           arg.rstrip[0..-2].to_f * dpi
         when /cm$/ # ends with "cm"
           arg.rstrip[0..-2].to_f * dpi * INCHES_IN_CM
+        when /mm$/ # ends with "mm"
+          arg.rstrip[0..-2].to_f * dpi * INCHES_IN_CM / 10.0
         else
           arg
         end