Strip trailing whitespace in separate commit, for clarity.
Update sample config file with syntax for enabling vgifts.

Because the cost of each vgift is set individually and stored in the database,
no pricing information is used in the config file.  Instead, the presence of
the vgifts key in the %SHOP hash is what activates the feature.

The bug spec says to expire vgifts after two weeks, but this seems like the
sort of decision that various sites might wish to treat differently.  So
make a new config parameter: $LJ::VGIFT_EXPIRE_DAYS (defaults to 14).

The vgift shop is obviously not ready to be turned on in production.
This syntax is being made available now to assist other developers who
are interested in testing the system as it evolves.