Additive fields

Modified the appender to work in two modes:
* FieldBehavior = Explicit - (default if property not set) that only writes the specified fields to mongoDb. This is how it currently works if you specify a field so you have to specify all the fields you want.
* FieldBehavior = Additive - Setting the FieldBehavior property to Additive will give you all the default fields (as if you specified none i.e. BackwardsCompatibility) plus any fields you specify. If you add a field with the same name as a default or previous field it will overwrite the previous one.

Specifying no fields in either behavior will give you the backwards compatible fields.

The motivation behind this change was less configuration. Less configuration. I get the defaults always so I don't miss anything crucial and only have to be explicit about the extra field needed.

Thanks for the appender!