Optimize components

- Use class attribute over className
- Code cleaning

src/atoms/Button.svelte

@@ -1,20 +1,19 @@
 <script>
     export let href = '#'
     export let type = 'a'
-    export let className = 'button'
     export let text = ''
 
     const hasSlot = arguments[1].$$slots !== undefined
 </script>
 
 {#if type === 'button'}
-<button class={className} class:button-icon={hasSlot} data-text={text} on:click>
+<button class={$$props.class ? $$props.class : 'button'} class:button-icon={hasSlot} data-text={text} on:click>
     <slot></slot>
     <span>{text}</span>
 </button>
 
 {:else}
-<a {href} class={className} class:button-icon={hasSlot} data-text={text} on:click>
+<a {href} class={$$props.class ? $$props.class : 'button'} class:button-icon={hasSlot} data-text={text} on:click>
     <slot></slot>
     <span>{text}</span>
 </a>