Added long buttons (closes #28)

Author: rugulous

demo/index.html

@@ -18,6 +18,7 @@ <h2 id="buttons">Buttons</h2>
 			<button class="btn-default">Default</button>
 			<button class="btn-error">Error</button>
 			<button class="btn-success">Success</button>
+			<button class="btn-dark btn-long">Big Black Button</button>
 
 			<hr />
 

scss/components/_buttons.scss

@@ -3,3 +3,9 @@
 		@include button-variant($colour);
 	}
 }
+
+.btn-long{
+	display: block;
+	margin: 0;
+	width: 100%;
+}