Provide a method to execute an external command without using a shell

The `Exec` task always spawns a shell to invoke the external command. This is convenient, but non-portable, insecure (though not exploitable here), and requires non-portable escaping.

A better solution is to provide a way to execute an external command without using a shell. It should have the following properties:
- Lists are respected.
- Any character can be passed to an external command. No manual escaping required.
- Special characters in property values do not cause problems.
- No shell is invoked, unless the external command _is_ the shell.
- It should be possible to pass multiple arguments to the shell
- The syntax should be something other than requiring a seperate XML element per argument.