Fixed a sys.platform check

So it turns out that on Linux, it sys.platform is "linux2" in Python 2 and "linux" in Python 3, so if doing a comparsion, need to do a .startswith() instead of a ==

Author: tleonhardt

cmd2.py

@@ -305,7 +305,7 @@ def new_func(instance, arg):
 
 # Can we access the clipboard, always true on Windows and Mac, but only sometimes on Linux
 can_clip = True
-if sys.platform == 'linux':
+if sys.platform.startswith('linux'):
     try:
         pyperclip.paste()
     except pyperclip.exceptions.PyperclipException: