fix(checkbox): expands the checkbox click radius

Closes #379.

Author: robertmesserle

src/components/checkbox/_checkbox.scss

@@ -15,6 +15,15 @@ md-checkbox {
     width: $checkbox-width;
     height: $checkbox-height;
 
+    &:after {
+      content: '';
+      position: absolute;
+      top: -10px;
+      right: -10px;
+      bottom: -10px;
+      left: -10px;
+    }
+
     .md-ripple-container {
       position: absolute;
       display: block;