Add dynamic var for JSON date format

Hello, dear Metosin,

Thank you for your wonderful library, it really helps me in my dealing with swagger-enabled APIs!
But there is one little issue that I'm having with date formats. It seems that `cheshire.core/generate-string` by default uses the format `"yyyy-MM-dd'T'HH:mm:ss'Z'"`, while officially it should be `"yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"`, according to this:
http://stackoverflow.com/questions/10286204/the-right-json-date-format.
So I made a small change to your code to make it possible for the user to change the date format to whatever he likes, while having the default one according to the standard.