Allow custom `YIELD` script for yielding sources

By default, this is just `cat`, which will simply pipe the source
content into the transformation, but one might change this to be
`cat doctype.dtd`, for example, to add a doctype to the beginning of
every source file, or a more complicated script.

`YIELD_DEPENDENCIES` can be used to specify dependencies of the `YIELD`
script to ensure the files are rebuilt when a dependency changes.