Update for Bootstrap 4

[Mark]

Hi there,

We are using spatie/laravel-menu and therefore spatie/menu in a project that is using Bootstrap 4. In Bootstrap 4 the markup for dropdowns changed, they are not using an <ul> for dropdowns anymore. We made quick fix to enable us to use a <div> instead of an <ul> and not wrap the links in an <li>.

If we create some tests and update the documentation accordingly, are you willing to merge this?

Thanks in advance 👍

[Sebastian De Deyne]

I'd like to add custom renderers in the future (see #62) but that's probably gonna take a while. In the mean time I'd accept PR that add's this, so I say go for it :)

[Maurice]

Thnx @sebastiandedeyne,

We are experiencing some issues with the 'active'-state as well now we have changed it for the dropdown items so we will investigate this as well.

[Mark]

Please see #64.