Merge pull request #431 from drosseau/master

Added not builtin

examples/not.ion

+not true || echo not true
+not false && echo not false
+true && not false && echo true and not false 
+not test -z "a" && echo not test
+not echo hello
+echo $?
\ No newline at end of file

examples/not.out

+not true
+not false
+true and not false
+not test
+hello
+-1

src/builtins/mod.rs

@@ -193,6 +193,20 @@ impl Builtin {
                         });
 
         /* Misc */
+        commands.insert("not",
+            Builtin {
+                name: "not",
+                help: "Reverses the exit status value of the given command.",
+                main: Box::new(|args: &[&str], shell: &mut Shell| -> i32 {
+                    let cmd = args[1..].join(" ");
+                    shell.on_command(&cmd);
+                    match shell.previous_status {
+                        SUCCESS => FAILURE,
+                        FAILURE => SUCCESS,
+                        _ => shell.previous_status
+                    }
+                }),
+            });
         commands.insert("set",
             Builtin {
                 name: "set",