New feature: Field mapper function
Per #59, new APIs around field mapper functions are added.
In many production environments, table column names are usually snake_case words, e.g. user_id, while we have to use CamelCase in struct types to make struct fields public and golint
happy. It's a bit redundant to use the db tag in every struct field. If there is a certain rule to map field names to table column names, We can use field mapper function to make code simpler.
See field mapper function sample as a demo.