references: add offset to pull

[Fishrock123]

Refs: https://github.com/Fishrock123/bob/issues/23

[Raynos]

If you add an offset to pull() it makes sense to also add an offset to next()

[Fishrock123]

@Raynos You mean to note where that particular read was done from?

[Raynos]

Correct, to note at which offset the n bytes where read into the buffer.

But this raises a good question, what does offset on pull() mean ? the offset to read into in the buffer or the offset to read from in whatever the source is ( like an offset in a file on disk ).

[Fishrock123]

It was supposed to be source read location. I am still not really sure this fits well into the stream abstraction...