Add absolute value to dict_int

I've seen a few requests on how to make FTS search on the absolute value of
integers. This question is usually driven by the fact that the text search
parser interprets a separating hyphen ("partnumber-987") as a minus sign.

There is currently no good answer for this that doesn't involve C coding.
I think this feature has a natural and trivial home in the dict_int
extension, so attached is a patch that does that.

There are no changes to the extension creation scripts, so there is no need
to bump the version. And I think the convention is that we don't bump the
version just to signify a change which implements a new feature when that
doesn't change the creation scripts.

Cheers,

Jeff