Add option to pass wkhtmltopdf options. Fix #176.

It seems that some time in the last couple years wkhtmltopdf's default
margins were changed from '10mm' to zero. As an alternative to #177,
this PR adds an option to pass in arbitrary wkhtmltopdf long arguments
and sets the default top and bottom margin to '10mm'.

src/verbs/build.coffee

@@ -176,6 +176,7 @@ _prep = ( src, dst, opts ) ->
   _opts.noTips = opts.noTips
   _opts.debug = opts.debug
   _opts.sort = opts.sort
+  _opts.wkhtmltopdf = opts.wkhtmltopdf
   that = @
 
   # Set up callbacks for internal generators