[2.x] Add HtmlString caster

[GrahamCampbell]

Before:

>>> Illuminate\Mail\Markdown::parse("### test\n");
=> Illuminate\Support\HtmlString {#3811}

After:

>>> Illuminate\Mail\Markdown::parse("### test\n");
=> Illuminate\Support\HtmlString {#4008
     html: "<h3>test</h3>\n",
   }