Avoid Windows command prompt popups when possible.

Before this commit some Windows users saw the command prompt pop
up briefly, and/or the taskbar flicker, every time the plugin ran.

Now the plugin will use xolox's vim-shell and vim-misc, if they are
available and we are on Windows, to execute external commands.  Xolox's
clever plugins avoid the command prompt popup and taskbar flicker.

Windows users with those plugins installed can opt out by setting a
variable in their vimrc.

Many thanks to @suxpert for the initial code.