compose: Support 'install-langs'

This should exist for the same reason the yum and RPM options do; some
people want to construct more minimal systems.
compose: Support 'remove-from-packages' entry

This is the equivalent of the 'removefrom' verb in Lorax's templating.
It's a lot more robust than a generic "rm-rf" type thing, because most
often you only want to remove files from particular packages.