add --watcher argument for runtests
add --watcher argument for runtests

This allows selecting a different watcher from the default automatic
selection.

kwlzn/watchman,dcolascione/laptop-watchman,nodakai/watchman,dcolascione/laptop-watchman,facebook/watchman,dcolascione/laptop-watchman,wez/watchman,facebook/watchman,wez/watchman,dhruvsinghal/watchman,dhruvsinghal/watchman,wez/watchman,dhruvsinghal/watchman,wez/watchman,wez/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,dhruvsinghal/watchman,nodakai/watchman,dcolascione/laptop-watchman,facebook/watchman,kwlzn/watchman,kwlzn/watchman,dcolascione/laptop-watchman,kwlzn/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,nodakai/watchman,wez/watchman,dcolascione/laptop-watchman,facebook/watchman,nodakai/watchman,dhruvsinghal/watchman,dhruvsinghal/watchman,wez/watchman,nodakai/watchman,facebook/watchman,kwlzn/watchman,dcolascione/laptop-watchman,facebook/watchman,kwlzn/watchman,dhruvsinghal/watchman,nodakai/watchman,facebook/watchman,dcolascione/laptop-watchman,wez/watchman,kwlzn/watchman,dhruvsinghal/watchman