First pass at refactoring shell command

Author: tleonhardt

cmd2.py

@@ -1143,7 +1143,16 @@ def do_shell(self, command):
         """Execute a command as if at the OS prompt.
 
     Usage:  shell <command> [arguments]"""
-        os.system(command)
+        try:
+            out = subprocess.check_output(shlex.split(command))
+        except subprocess.CalledProcessError as e:
+            self.perror(e, traceback_war=False)
+        except FileNotFoundError as e:
+            self.perror(e, traceback_war=False)
+        else:
+            if six.PY3:
+                out = out.decode()
+            self.stdout.write(out + '\n')
 
     def path_complete(self, text, line, begidx, endidx, dir_exe_only=False, dir_only=False):
         """Method called to complete an input line by local file system path completion.