Sync menu theme with menu-items by default

Close #861.
It's already done:
https://github.com/bem/bem-components/blob/ba59063/common.blocks/menu/menu.tmpl-specs/10-default.bemjson.js
https://github.com/bem/bem-components/blob/ba59063/common.blocks/menu/menu.tmpl-specs/10-default.html

I've just did some refactoring.
`var mods = this.mods;`