removeStore = (storeName, key = null) => {
  if (localStorage.getItem(storeName)) {
    if (!key) {
      localStorage.removeItem(storeName);
      return { success: true, message: 'store removed.' };
    }
    const obj = JSON.parse(localStorage.getItem(storeName));
    delete obj[key];
    localStorage.setItem(storeName, JSON.stringify(obj));
    return { success: true, message: 'key from store removed.' };
  }
  return { success: false, message: 'store not found.' };
}
Remove the given key 'or' storeName from the localStorage. If the key is null
then the store is removed.
@param  {String} storeName  the name of the store.
@param  {String} [key=null] the name of the key in the store. Null by default.
@return {Object}            {success: boolean, message: 'string'}