Allow opting out of ES3SafeFilter.

We were forced to move the `ES3SafeFilter` step to only operate on
transpiled ES6 output (since it fails on some ES6 module syntax).

Sadly, this means that we are reprocessing the ENTRIE app/ tree for
every rebuild.  Which is **SLOW**!!
Merge pull request #966 from rjackson/allow-opting-out-of-es3safe

Allow opting out of ES3SafeFilter.
Make sure the build traps `SIGINT` and `SIGTERM` for cleanup.
Merge pull request #971 from WMeldon/sigint-trapping

Make sure the build traps `SIGINT` and `SIGTERM` for cleanup.