Support equivalent of `blank` `nrformats`

[dominikh]

The built-in CTRL-A supports the following option in nrformats:

blank	If included, treat numbers as signed or unsigned based on
	preceding whitespace.  If a number with a leading dash has its
	dash immediately preceded by a non-whitespace character (i.e.,
	not a tab or a " "), the negative sign won't be considered as
	part of the number. 

It would be nice if augend.integer supported this mode, instead of having to use different values for natural for different scenarios.