Fix addons with Bootstrap 4

[tortuetorche]

Usage with Laravel:

{{-- Basic text addon --}}
{!! Former::open()->method('GET') !!}
    {!! Former::text('test2')->append('€') !!}
{!! Former::close() !!}

{{-- Icon addon --}}
{!! Former::open()->method('GET') !!}
    {!! Former::text('test icon')->append('<i class="fa fa-thumbs-up"></i>') !!}
{!! Former::close() !!}

{{-- Multiple text addons --}}
{!! Former::open()->method('GET') !!}
    {!! Former::text('test3')->append(['&euro;', 'second']) !!}
{!! Former::close() !!}

{{-- Button addon --}}
{!! Former::open()->method('GET') !!}
  {!! Former::text('test4')->append('<button class="btn">OK</button>') !!}
{!! Former::close() !!}

{{-- Button addons --}}
{!! Former::open()->method('GET') !!}
  {!! Former::text('test5')->append(['<button class="btn">OK</button>', '<button class="btn">2</button>']) !!}
{!! Former::close() !!}

Credits: @mrextreme

Reference: https://github.com/formers/former/issues/581#issuecomment-473243405