Implement Shell(prefix_with=[arg1, arg2])

[jroose]

As an alternative to the built-in 'user=' implementation I should add an optional Shell() instance parameter called prefix_with() that supplies arguments to be prefixed before running commands. For example this could be useful as an alternative implementation to the 'user=' argument to sh.command() that prefixes commands with sudo. It could also come in handy when working with chroot environments by causing all commands inside a specific Shell() instance to run within the chroot by prefixing commands with ['chroot', mount_point].