add a `SaveWith` function that accepts `SaveOptions` as a parameter, letting you tweak things like whether timestamps will be set before an insert or update operation
- move internal model helper funcs into model_internals.go
- add Swap method to Model
- modify `getNested` util to support indexing slice fields such as `Abc[0].Def`
- add support for `autoinc` struct tag
- add Pull method to Model
- add utility func to increment a value of an unknown type by 1
- idcounter.go no longer increments the counter value (it is now up to the caller to increment the returned value themselves)
