maktaba#system#Call() should capture stdout and stderr even when mode is 'foreground'

[dbarnett]

maktaba#system#Call captures stdout and stderr output so vim plugins can process it programmatically, but that's only implemented for 'hidden' mode. It would be useful to be able to capture output from foreground mode, in addition to having it displayed on the screen.

This might require some kind of hack possibly using GNU tee.