New utilities
Support for chained rules.

These will be useful if there are a "pipeline" of targets, but the
intermediate steps aren't really interesting enough to warrant their
own target names.  See tests/testthatain.yml for the syntax: instead
of having sections 'rule' and 'depends', a target can contain a
section 'chained'.  That section is an *ordered* list, each
element of which is a yaml dictionary with elements rule, depends,
cleanup_level.  This builds a series of implicit targets.
Simplify creation of implicit file targets
Make optparse optional
Get stricter about makerfiles being out of the current directory

This is stricter than needed but gives flexibility for changing
later.
Update README with installation instructions