Added bootstrap.classes.thead option

Author: ConnorHowell

resources/views/bootstrap-4/includes/thead.blade.php

@@ -1,5 +1,5 @@
 @if ($tableHeaderEnabled)
-    <thead>
+    <thead class="{{ $this->getOption('bootstrap.classes.thead') }}">
         @include('laravel-livewire-tables::'.config('laravel-livewire-tables.theme').'.includes.columns')
     </thead>
 @endif