doc/treefile.md: Document remove-files
compose: Rename _postprocess function to _prepare_rootfs_for_commit()

It's a lot clearer what we're doing.  This is preparation for further
work in cleaning up how we do content processing.
compose: Split out JSON helpers into separate file

In preparation for using some of them in the postprocessing code too.

Yeah, parsing JSON in C sucks =( Oh well.
compose: Move content processing code into -postprocess

It seems clearer to me if all code which is operating on the content
(such as enabling systemd units) is under -postprocess.  The "compose
tree" code should be tying everything together and calling out to
sub-components.

This is prep for adding more postprocessing code.
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.