Toggles don't trigger `is-focused` class on focus

[tzemanovic]

The issue can be seen on the Toggle demo page:

https://debois.github.io/elm-mdl/#toggles

When you inspect the elements and set focus on them with tab key, none of the Toggle types get applied is-focused class, so the focus is not visible.

There's a related issue upstream: google/material-design-lite#5105, but the JS problem mentioned there obviously doesn't apply to this library, though the CSS problem does propage here.