Fixes: #292 #221

This PR would allow users to map one theme to multiple custom classes.

The value can now accept a string or string[] as value.

if we want to map a theme to multiple custom theme values, we can simply pass an array:

<ThemeProvider attribute='class' value={{ pink: ['my-pink-theme', 'another-pink-theme'] }}>

When the selected theme is pink, both my-pink-theme and another-pink-theme will be added as class names.