More flexible handling of expires option

It would be nice to be able to just use {expires: "never"} or a specific date, and let amplify store figure out the details.   My proposed changes allow for this via a small function that checks if expires is a string and then does the appropriate conversion to a millisecond offset.  I used a function for this to keep the existing code "clean" and allow for other possible keywords for "expires" to be added to the function.