implement indextargets option 'DefaultEnabled'

implement indextargets option 'DefaultEnabled'

Some targets like Contents-udeb are special-needs targets. Shipping the
configuration snippet for them is okay, but they shouldn't be downloaded
by default. Forcing the user to enable targets by uncommenting targets
is wrong and this would still not really solve the problem completely as
even if you want to download some -udebs it will probably not be for all
sources you have enabled, so having the possibility of disabling a
target by default, but giving the user the option to enable it on a
per-source entry basis is better.