Fix for extra output at startup when using Python 2.7 on Linux

[tleonhardt]

Fix a bug where an extra line of output was printing at startup when using Python 2.7 on Linux where Pyperclip was using xclip under the hood and the clipboard was empty.

The fix involves using a different way of testing to make sure pyperclip's cut/paste functionality is working. So on Linux, cmd2 applications will now copy and empty string to the clipboard at startup just to make sure they can.

Hopefully this won't inconvenience users. It seems better than printing a line at application startup which won't make any sense to end users.

[codecov]

Codecov Report

Merging #202 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #202   +/-   ##
======================================
  Coverage    96.8%   96.8%           
======================================
  Files           1       1           
  Lines        1158    1158           
======================================
  Hits         1121    1121           
  Misses         37      37

Impacted Files	Coverage Δ
cmd2.py	96.8% <100%> (ø)	⬆️

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