Added custom capabilities

I replaced the single capability (`manage_options`) used by the plugin by 2 custom ones:

* `nwm_edit_route`, granted to __editor__ role by default, for all route related actions
* `nwm_manage_options`, granted to __administrator__ role by default, for everything under settings

This allows the administrator to fine tune their capabilities using plugins such as [User Role Editor](https://wordpress.org/plugins/user-role-editor/).