Optionally add attributes

[ironfroggy]

Some attributes trigger behaviors and meanings simply by their presence, such as the readonly attribute. In these cases even an empty string passed to render_field will cause the field to have the readonly attribute and be read-only, so there is no way to do this and pass 'false' in any form.

In the way that render_field supports both = and += to set or append an attribute, a syntax to optionally add would be great. OR, explicit support for fields like readonly that would just ignore the attribute if the value given was false.

[kmike]

I like the idea. Something like {% render_field form.text readonly?=value %}?
Pull requests are welcome.

[svengt]

The pull request from @Kos looks pretty good! Any chance this will be merged soon?