Fix Exec to handle quotes

[bitfield]

Issue #32 highlighted a problem with the simple approach (strings.Fields) to splitting a command string into a suitable slice of strings for exec.Command. A program like this:

p := script.Exec("bash -c 'docker ps'")

didn't work as expected, because the actual slice of strings passed to the command was:

{"bash", "-c", "'docker", "ps'"}

which causes the shell to choke. We need a slightly smarter approach which takes into account quoted strings, and doesn't split them:

{"bash", "-c", "docker ps"}

The creachadair/shell library provides a facility (Split) to do exactly this. Thanks to @shumin1027 for the original report, @posener for input, and @jrswab for an initial draft PR on this.