Allow github support to be disabled in book.json
Allow github support to be disabled in book.json

Fixes #238

ferrior30/gitbook,codepiano/gitbook,xcv58/gitbook,hujianfei1989/gitbook,npracht/documentation,sudobashme/gitbook,thelastmile/gitbook,jocr1627/gitbook,FKV587/gitbook,snowsnail/gitbook,codepiano/gitbook,a-moses/gitbook,iamchenxin/gitbook,anrim/gitbook,bjlxj2008/gitbook,gaearon/gitbook,Keystion/gitbook,gencer/gitbook,intfrr/gitbook,2390183798/gitbook,bradparks/gitbook,CN-Sean/gitbook,jasonslyvia/gitbook,escopecz/documentation,tzq668766/gitbook,minghe/gitbook,shibe97/gitbook,npracht/documentation,xxxhycl2010/gitbook,FKV587/gitbook,intfrr/gitbook,switchspan/gitbook,justinleoye/gitbook,iflyup/gitbook,gaearon/gitbook,rohan07/gitbook,qingying5810/gitbook,haamop/documentation,xxxhycl2010/gitbook,Abhikos/gitbook,jocr1627/gitbook,tzq668766/gitbook,iflyup/gitbook,bradparks/gitbook,vehar/gitbook,mruse/gitbook,boyXiong/gitbook,grokcoder/gitbook,iamchenxin/gitbook,justinleoye/gitbook,guiquanz/gitbook,hujianfei1989/gitbook,grokcoder/gitbook,vehar/gitbook,strawluffy/gitbook,xiongjungit/gitbook,gdbooks/gitbook,qingying5810/gitbook,nycitt/gitbook,JohnTroony/gitbook,xcv58/gitbook,sunlianghua/gitbook,tshoper/gitbook,palerdot/gitbook,2390183798/gitbook,jasonslyvia/gitbook,yaonphy/SwiftBlog,shibe97/gitbook,mautic/documentation,a-moses/gitbook,JohnTroony/gitbook,GitbookIO/gitbook,JozoVilcek/gitbook,mruse/gitbook,OriPekelman/gitbook,minghe/gitbook,wewelove/gitbook,youprofit/gitbook,lucciano/gitbook,athiruban/gitbook,sudobashme/gitbook,hongbinz/gitbook,sunlianghua/gitbook,webwlsong/gitbook,kamyu104/gitbook,palerdot/gitbook,ryanswanson/gitbook,switchspan/gitbook,kamyu104/gitbook,OriPekelman/gitbook,ZachLamb/gitbook,megumiteam/documentation,mautic/documentation,snowsnail/gitbook,bjlxj2008/gitbook,guiquanz/gitbook,webwlsong/gitbook,gdbooks/gitbook,athiruban/gitbook,rohan07/gitbook,escopecz/documentation,tshoper/gitbook,CN-Sean/gitbook,gencer/gitbook,lucciano/gitbook,xiongjungit/gitbook,ZachLamb/gitbook,ferrior30/gitbook,hongbinz/gitbook,thelastmile/gitbook,youprofit/gitbook,wewelove/gitbook,nycitt/gitbook,alex-dixon/gitbook,megumiteam/documentation,alex-dixon/gitbook,haamop/documentation,boyXiong/gitbook