Buffered commands: Relative jump

[stevesims]

Is it useful to add in "relative" jump functionality?

The inclusion of this functionality was suggested in #51

This could/would allow for jumping solely within a buffer, supporting jumping backwards/forwards a fixed number of either blocks, or bytes.

This could have some uses... But is it useful enough to warrant its inclusion?

Currently we only support jumping to absolute positions. Arguably that is enough...

One slight complication with adding this in would be that it wouldn't really fit with the the shape of the current buffered commands API, as such a command explicitly shouldn't specify the buffer, as it's inherently "this buffer".