Reimplement offset() and add range().

[felix9]

offsetEnd() is new.

offset() existed prior to 0.10, and the existing name is why I'm not
calling it offsetStart().

Fixes #526

[jaubourg]

You could name it offsetStart and alias it as offset for backward compatibility. Best of both worlds.

[felix9]

Sure, but I lean slightly in favor of avoiding unnecessary redundancy. I'll add the alias if anyone feels strongly about it

[futagoza]

@felix9 I was planning on adding range() instead of offsetEnd() as the reason most people look for the ending offset is when making a range:

function range() {
    return [peg$savedPos, peg$currPos];
}

Any particular reason (and/or use case) you chose to use offsetEnd() instead?

[felix9]

No reason. Range is probably better, yeah.