Ability to change `sub-menu`

[nicholasio]

Is your feature request related to a problem? Please describe. I noticed there is hard coded sub-menu class used in the nav component. Could this be part of settings so it could be changed to something else.

This would enable changing sub-menu class to something else that fits better for chosen naming convention.

Describe the solution you'd like sub-menu can be default but this could be overwritten via settings.