Allow adding CSS classes to generated HTML elements

[gflohr]

Checklist

• I am running the latest version.
• I have checked the troubleshooting guide
• I have searched the open issues, but my issue has not already been reported

What is not working?

I want to style elements not with the class names that isso provides but my own ones. For example, I want the submit button to have the classes "btn btn-primary". That way, I can use a CSS framework, for example Bootstrap which defines tons of rules for these classes, so that just copying the CSS is not really an option.

Several ideas how this could be implemented:

• use a template for the generated HTML
• fire an event after the comments section has been rendered, so that the DOM can be manipulated
• allow adding class names to the generated HTML elements

[ix5]

Might be possible to implement this using the data-isso-* attributes similar to translation overrides.

Features like this (especially hooking into some event handler) would require a re-thinking of how the client currently works. Feel free to propose a better extension/hooking mechanism than the current state.

See also #894