Custom capabilities for fine permission tuning

I created a custom capability for each menu item so that users can fine tune their permissions with plugins like [User Role Editor](https://wordpress.org/plugins/user-role-editor/).


__Summary of the custom capabilities:__

Capability name | Roles assigned to (by default) | Description
----------------|--------------------------------|------------
`nwm_edit_route` | Administrator, Editor | Add, edit or remove points on the map
`nwm_manage_maps` | Administrator, Editor | Add, edit or remove maps
`nwm_read_faq` | Administrator, Editor | Read the FAQ
`nwm_manage_options` | Administrator | Adjust the plugin's settings