Use more specific class name, and only add it for JS fallback elements

[felixarntz]

Summary

This PR can be summarized in the following changelog entry:

• Improve compatibility with other plugins by using more specific class and only adding it for JS fallback.

Fixes #4

Checklist:

• [x] My code is tested.
• [x] My code is backward-compatible with WordPress 4.7 and PHP 7.0.
• [x] My code follows the WordPress coding standards.
• [x] My code has proper inline documentation.

[felixarntz]

This will need to have a merge conflict resolved once #8 is merged.