make Watching.close callback optional
make Watching.close callback optional

https://github.com/webpack/webpack/issues/769#issuecomment-73615872
ts-webpack/webpack,Ju2ender/webpack,nkzawa/webpack,zongkelong/webpack,WenjieWu/webpack,advisory/webpack,izaakschroeder/webpack,mjackson/webpack,Faiz7412/webpack,webpack/webpack,silvaemota/webpack,cgvarela/webpack,mjackson/webpack,hubli/webpack,stephan83/webpack,giobero999/webpack,imjerrybao/webpack,jakecraige/webpack,ts-webpack/webpack,thaiat/webpack,Cavitt/webpack,cococoder/webpack,hubli/webpack,tiagoengel/webpack,voldern/webpack,g0ddish/webpack,VANITAX/webpack,bjlxj2008/webpack,EliteScientist/webpack,spalger/webpack,skyh/webpack,mcanthony/webpack,benmosher/webpack,g0ddish/webpack,longjl/webpack,mvandiest/webpack,jescalan/webpack,rlugojr/webpack,jescalan/webpack,KamilSzot/webpack,VANITAX/webpack,jshosomichi/webpack,keyzf/webpack,xuechunL/webpack,cococoder/webpack,advisory/webpack,marcello3d/webpack,widgetic/webpack,smartpcr/webpack,ifandelse/webpack,matthewlehner/webpack,modulexcite/webpack,arielschiavoni/webpack,wjb12/webpack,tiagoengel/webpack,mvayngrib/webpack,xatax/webpack,mvayngrib/webpack,g6scheme/webpack,nkzawa/webpack,mcanthony/webpack,neo15201788751/webpack-1,xuechunL/webpack,NekR/webpack,chentsulin/webpack,jamestautges/webpack,spalger/webpack,dncrews/webpack,longjl/webpack,ali/webpack,rpora/webpack,s-panferov/webpack,shaunstanislaus/webpack,2947721120/webpack,michael-gabenna/webpack,Jonekee/webpack,loganlinn/webpack,Teino1978-Corp/Teino1978-Corp-webpack-bundled-assets,jescalan/webpack,haolee1990/webpack,Jonekee/webpack,DouglasHeriot/webpack,djchie/webpack,xatax/webpack,skyh/webpack,Dashed/webpack,ts-webpack/webpack,michael-gabenna/webpack,ifandelse/webpack,SimenB/webpack,g0ddish/webpack,ianwcarlson/webpack,hcxiong/webpack,pugnascotia/webpack,jamestautges/webpack,EliteScientist/webpack,Teino1978-Corp/Teino1978-Corp-webpack-bundled-assets,giobero999/webpack,zenlambda/webpack,cesarandreu/webpack,mdcone/webpack,loganlinn/webpack,widgetic/webpack,smartpcr/webpack,Ju2ender/webpack,pugnascotia/webpack,dncrews/webpack,mishk0/webpack,akx/webpack,2947721120/webpack,quentinyang/webpack,cranca/webpack,plasticine/webpack,SimenB/webpack,rlugojr/webpack,keyzf/webpack,ftdebugger/webpack,jerry-he/webpack,NekR/webpack,rlugojr/webpack,rpora/webpack,Zeratul5/webpack,zyj1022/webpack,sdawson/webpack,beni55/webpack,Dashed/webpack,quentinyang/webpack,neo15201788751/webpack-1,wjb12/webpack,ts-webpack/webpack,honger05/webpack,modulexcite/webpack,benmosher/webpack,djchie/webpack,yisbug/webpack,izaakschroeder/webpack,WenjieWu/webpack,mvandiest/webpack,bjlxj2008/webpack,webpack/webpack,ianwcarlson/webpack,matthewlehner/webpack,webpack/webpack,Zeratul5/webpack,marcello3d/webpack,fomenyesu/webpack,cgvarela/webpack,webpack/webpack,zyj1022/webpack,tsu-complete/webpack,philipdai/webpack,chentsulin/webpack,ftdebugger/webpack,rf-/webpack,ali/webpack,jshosomichi/webpack,jerry-he/webpack,akx/webpack,pugnascotia/webpack,halfzebra/webpack,haolee1990/webpack,fomenyesu/webpack,stephan83/webpack,imjerrybao/webpack,Cavitt/webpack,ZECTBynmo/webpack,voldern/webpack,philipdai/webpack,cranca/webpack,nagyistoce/webpack,silvaemota/webpack,halfzebra/webpack,plasticine/webpack,xiaoking/webpack,yisbug/webpack,xiaoking/webpack,nagyistoce/webpack,arielschiavoni/webpack,mishk0/webpack,zenlambda/webpack,nkzawa/webpack,cesarandreu/webpack,DouglasHeriot/webpack,s-panferov/webpack,hcxiong/webpack,ywjrg/webpack,ZECTBynmo/webpack,shaunstanislaus/webpack,honger05/webpack,mdcone/webpack,halfzebra/webpack,ywjrg/webpack,KamilSzot/webpack,sdawson/webpack,jakecraige/webpack,Faiz7412/webpack,thaiat/webpack,cesarandreu/webpack,g6scheme/webpack,zongkelong/webpack,SimenB/webpack,SimenB/webpack,tsu-complete/webpack,rf-/webpack,beni55/webpack