support explain for model
support explain for model

maxzhang/thinkjs,akira-cn/thinkjs,akira-cn/thinkjs,snadn/thinkjs,maxzhang/thinkjs,maxzhang/thinkjs,Qihoo360/thinkjs,thinkjs/thinkjs,75team/thinkjs,75team/thinkjs,akira-cn/thinkjs,75team/thinkjs