Manual specification of default target.

Because the configuration is a a yaml map, the sort is not
guaranteed stable!  So the first target can't be used.  The default
target is worked out in this order of preference:
1. a top-level key `target_default` in the makerfile
2. a target called "all" (following make's convention)
3. there is no default target and maker will generate an error if
$make() is called without a target.
Drop json in favour of rds for the database

For now at least, it's just simpler if I can rely (to some
degree) on map ordering.  There are still issues with this
though (see test-manual.R)