New language feature: changed_when


changed_when can be used to decide when to trigger a handler based on the output or return code of a shell command. It is similar to failed_when like I previously posted about

Let me know if you think other language features other than resources and push/pull modes are needed. I'm trying to keep things pretty minimal, though it's common enough to want to corral the output of shell commands, so that's why these are there.