add mapper configuration

[rudychen93]

Sometimes I don't need @Mapper annotation, so I add a configuration called mapper to decide if I need to add @Mapper annotations.

[moonciki]

This pr is helpful to me.
In the case of multiple data sources, I use configurable MapperScannerConfigurer to initialize mappers, if use @Mapper annotation, there will happen Error when startup. So please apply this pr. Thank you.

[ghost]

@softwareloop Could you merge this PR? Mybatis Generator with ANNOTATEDMAPPER config adds @ Mapper annotation by default. This Plugin adds another @ Mapper annotation. JDK fails to compile generated files, because only 1 annotation can exist.