fallback for browsers that don't support contain property

Please see https://github.com/ruphin/slidem/issues/4#issuecomment-480543521

tl;dr: zero-md should add `overflow:auto` before `contain: content` in `_buildStyles` to support browsers that don't have the `contain` CSS property.

