Ensure outdated/removed/zombie outputs are deleted

Given a rule like:

```
build/%{1}: cmd/*
inputs: **/*.go
build: go build -o %{OUT} -o build/%{1}
```

If a `cmd` is removed or renamed between builds, the `build` directory will contain stale outputs. The Bit database could be used to track this . This should be relatively straightforward, in that we could populate a variable with a list of files that have been removed from the build since the last run.

eg.

```
build/%{1}: cmd/*
inputs: **/*.go
build:
for stale in %{STALE}; do [[ $stale == build/* ]] && rm $stale; done
go build -o %{OUT} -o build/%{1}
```

Shake has the concept of ["pruning"](http://neilmitchell.blogspot.com/2015/04/cleaning-stale-files-with-shake.html), which could be useful inspiration.