Fix #2, Preserve environment boolean.

* This provides the capability to preserve the environment for the child
  process from that which is passed in to Sprok itself.
* The tests are kind of brittle (checking some shell command?). As far
  as I can tell, the shell command is not used by anything bar tests.
Probably something bad, haha. I believe in this case they'd be duped. Should probably not allow dupes, or permissively allow dupes to be clobbered by p.Env.