`font-size: 1rem` for the popup, use `em` for all popup children

@acurrieclark's suggestion from #25 (comment)

A better option here might have been to use em with a higher element given an rem factor. This way it is easy to override the base value with one css setting.

There's only one CSS setting (SCSS var) now 🎉

$swal2-font-size: 1rem;