Toggle input style missing size

[J-Kallunki]

Should base on this format:

<label for="suomifi-checkbox-id" class=“suomifi-checkbox">
  <input type="checkbox" id="suomifi-checkbox-id" class=“suomifi-checkbox_input" />
  <svg class=“suomifi-checkbox_icon" />
  <span>Label for the checkbox</span>
</label>

.suomifi-checkbox .suomifi-checkbox_input:checked + .suomifi-checkbox_icon {
}
.suomifi-checkbox .suomifi-checkbox_input:focus + .suomifi-checkbox_icon {
}
.suomifi-checkbox .suomifi-checkbox_input {
  position: absolute;
  // SVG-ikcon size
  width: 1em;
  height: 1em;
  opacity: 0;
}

and use idGenerator()

Originally posted by @J-Kallunki in #154 (comment)