Cleanup clip and paste buffer functions

[kotfu]

There is a global variable in cmd2.py::disable_clip() which we need to figure out how to get rid of, and the cmd2.py::get_paste_buffer() and cmd2.py::write_to_paste_buffer() functions should probably be moved to utils.py

[tleonhardt]

The global can_clip variable exists as a global boolean flag to reflect whether or not the pyperclip module has loaded correctly and we thus have proper access to clipboard/pastebuffer cut-and-paste functionality. It is only used in two places:

1. cmd2.py: cmd2.Cmd._redirect_output()
2. test_cmd2.py: test_send_to_paste_buffer()

I don't see any reason why it couldn't become a boolean instance variable within cmd2.Cmd. If we could 100% trust pyperclip to always work all the time on all versions of that module, Python, and OSes then we wouldn't need this variable at all. But in the past we had some issues when new versions of pyperclip come out, so this is intended to help cmd2 remain functional just without that one feature if/when that happens.

Moving those two functions to utils.py should be straightforward.

[kotfu]

When you wrote "I don't see any reason why it could become..." did you mean "I don't see any reason why it couldn't become..."? If so, I think an instance boolean is much better than a global.

I wasn't really clear in my original comment, I'm not advocating getting rid of the logic of testing whether pyperclip works, just figuring out a way to retain the logic without using a global variable.

[tleonhardt]

Yeah, you have the right of it. I was tired and mistyped.