Allow appending neighbor entries, which is useful with certain kernel drivers

[mepholic]

This is my attempt at fixing issue #60

I've left more commentary on the issue, and spent some time reading kernel sources. I think this solution should be sufficient for now, but I don't know if my approach at making REPLACE override APPEND if both are specified is a good idea. I wasn't sure what the project's guidelines were on error handling, and if generating an error from the library itself was acceptable, or if having netlink complain (or possibly silently do something weird) is preferred.

I've tested the code for my particular use case, and it works great!

If anybody has any thoughts on this implementation, I'd be happy to attempt to solve this another way. 😀

[cathay4t]

Please do similar stuff like #57 by using a builder.

The builder approach is collecting all configurations and generate the final netlink message at final call of build(), it is better than current approach on modifying internal stored netlink message.