Fix Button styling

src/components/atoms/Button.svelte

@@ -2,15 +2,20 @@
     export let text: string
     export let tag: string = 'a'
     export let url: string = undefined
+
+    const classes = [
+        'button',
+        $$props.class
+    ].join(' ')
 </script>
 
 {#if tag === 'button'}
-    <button class="button {$$props.class}">
+    <button class={classes}>
         <slot />
         {text}
     </button>
 {:else if tag === 'a'}
-    <a href={url} class="button {$$props.class}">
+    <a href={url} class={classes}>
         <slot />
         <span>{text}</span>
     </a>