Support for submenu

[viankakrisna]

Now it only print the submenu beside the parent menu. Maybe we can implement something like https://www.getmdl.io/components/index.html#menus-section, triggered on hover?

sub-menu in footer also broken:

[braginteractive]

MDL doesn't have dropdown menus for any of the navigation layouts https://www.getmdl.io/components/index.html#layout-section

This would be a feature request for MDL https://github.com/google/material-design-lite