Expose os/exec.Cmd.Env functionality

[robothor]

Hi! Thanks for starting this tool!

We needed to expose the ability to set environment variables on the underlying Cmd object. This PR contains the minimal set of changes we needed to get this to work and a basic test for this functionality.

As per the normal os/exec.Cmd, if the Env is nil (i.e. not set) then the current process' environment will be used.

I didn't see any contributor guidelines, so this is best-effort -- please let me know if you need me to make any changes.

[coveralls]

Coverage remained the same at 100.0% when pulling 924ae7f on robothor:env into bcc8acb on go-cmd:master.

[daniel-nichter]

It maintains backwards-compatibility, so I'll merge it. It makes me realize that I'll need to redesign this pkg a little and release a 1.1 series. Question is: what if someone wants to set/change Cmd.Dir or Cmd.ExtraFiles in the base os/exec.Cmd? This pkg can't and shouldn't duplicate all the base Cmd fields, but it shouldn't block the user form setting/changing them, either. So a better design for v1.1 would be to expose the base Cmd directly so users have full control of that while still getting the benefits of this wrapper.

That can wait for later. For now, adding Env is an acceptable duplication of the base Cmd.Env, and test coverage remains 100%. Thanks for the PR!