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Disable showcmd

This is causing noticeable slowdowns on OS X terminals (Terminal.app *and*
iTerm2, see https://github.com/tpope/vim-sensible/issues/49).

Also, blinking commands (particularly noticeable with repeating line motions
(hjkl)) is distracting.

`showcmd`'s principle use (for me) is a line count during visual selection.
More than happy to compromise in order to resolve the above (selection line
counts are easily achievable in other ways, e.g. `g^G`).