Fix string split in command.Run(), use strings.Fields() instead of strin...

...gs.Split()

Fixes a panic on the `devel` branch, seen here in tests: https://gist.github.com/mdlayher/87bd542351268dbd6bb4

I believe the issue is that the command output contains whitespace characters which are not just tabs, and as a result, the input was not being split properly.

This appears to resolve the issue in the tests, as well as in my project.