Add fallback

[franciscop]

Add a function to allow for a fallback (possibly with localStorage):

cookies.fallback = function(obj, opts){
  if (typeof obj !== 'string') {
    console.log("Storing the data with the fallback method");
  } else {
    console.log("Retrieving the data with the fallback method");
  }
  storage(obj, opts);
}

cookies({
  token: "a string over 10kb in size that doesn't fit in the cookies"
});

It cannot default to it (as in, we cannot create and auto-integrate localStorage into cookies for example) since when setting cookies it should set the cookie or fail.

[franciscop]

This function could be invoked if trying to set a too large cookie with no fallback:

cookies.error = function(error, data, opt){
  throw error;
}

And in other situations where an error can exist:

• string instead of an object for the options
• Invalid option values: string instead of the valid expires fields, not string for domain, etc.
• Cookie couldn't be set (maybe too large)