TODO.md

Short-term

* Expand ${foo}.
* Unit tests.
* Expanding regex matches in targets.
* Dummy rule for multiple explicit targets
* Expand `$newprereq`.
* Expand `$alltargets`.
* Man page.
* Namelist syntax.
* Environment variables.

Long-term

* Nicer syntax for alternative-shell rules.
* An attribute to demand n processors for a particular rule. This way
  resource hog rules can be run on their own without disabling parallel
  make.