prepare the snaplet for more backends

I include in this small pull-request the steps discussed in [Issue 2](https://github.com/Soostone/snaplet-persistent/issues/2)
I don't know if this kindof magic-string is the right way to do it - for now I stick to it to not break even more, but I think we should either make these two configurations explicite (by passing them in) or rename them to just `con-str` and `pool-size`, because for the sqlite backend I now have different names (`sqlite-con-str` and `sqlite-pool-size`)