Support sld files

[KeenS]

In my understanding, how to load sld files is up to implementation. Thus let's put sld under certain directory and compile into binary.

[nyuichi]

@KeenS

Well, I don't like the idea that changing behavior depending on file extension. If I misunderstand you, that is you are talking about changing extensions of our scheme library files under piclib/ and contrib, I think I can try.