Fix unknown fields warning when loading makerfile
Simpler version of installing runner script
Invisibly return objects when making them
Convenience wrapper function "make"

This should make it heaps easier to get going with things, not
needing to deal with crazy reference objects when you only
want to quickly build something.
Helper functions for creating plots

No need to manually call pdf() to create outputs.  Instead,
add a section `plot: true` to the target definition and
the rule only has to take care of making the plot.  Keys
can be specified so that writing `plot: {width: 6}` will
pass the width through.  The device type is determined from the
target file extension, though only pdf and png are supported until
I work out how to deal with Cairo devices.
Generate plot of the dependency graph.

This is just a proof of concept!
Refactor 'targets' to be self contained.

There's a slightly artificial distinction now between "active" and
"inactive" targets, but it's just for bookkeeping.  The maker instance
passes itself through to each target so that they can run themselves.
There's a bit more tidying to go, but this isolates all of the major
switching between behaviours into the targets.R file and class.