Nil-pointers fixes

v1.1.1 Fixes marshaling and unmarshaling pointer fields with nil value. Marshaling was straight panicking, while unmarshaling was setting zero-values when unwanted (i.e., omitempty was not specified).

Now redmap marshals nil-pointers with the underlying type's zero value. While unmarshaling, redmap sets the underlying type's zero value to pointer fields with no key found in the input map, unless omitempty is specified. In that case, the pointer field is left with the nil value.

Custom interface

Added StringMapMarshaler and StringMapUnmarshaler to allow to customize how a struct or a non-struct type is converted into and from a map of strings.

Initial release

Basic functionality is implemented, with a decent test coverage. Expect more features coming in the following months.