Offsets for `Get` functions

I was porting some `conduit` code to `pipes` when I stepped into [sourceHandleRange](http://hackage.haskell.org/package/conduit-1.0.17.1/docs/Data-Conduit-Binary.html#v:sourceHandleRange) which doesn't have an equivalent in `pipes-bytestring`.

If you like having these `...Range` functions in `pipes-bytestring` I could add the rest of them to this pull request (`hGetSomeRange`, `hGetRangeN`...). Otherwise, I can easily define those functions directly where I need them, so it's not a big issue for me not having them in `pipes-bytestring`.