go.tools/blog: strip prefix when serving static content
go.tools/blog: strip prefix when serving static content

This fix permits godoc to serve images correctly under /blog/.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/22700043

jdhenke/tools,runcom/tools,sshetty10/tools,glasser/golang-tools,nicot/tools,cw2018/tools,josharian/tools,cmars/tools,maddyonline/tools,magiccao/tools,cmars/tools,glasser/golang-tools,golang/tools,4ad/tools,argandas/tools,springning/tools,sshetty10/tools,maddyonline/tools,josharian/tools,dmitris/tools,glasser/golang-tools,magiccao/tools,golang/tools,magiccao/tools,dmitris/tools,matryer/tools,runcom/tools,cw2018/tools,jdhenke/tools,syreclabs/go-tools,cw2018/tools,maddyonline/tools,matryer/tools,runcom/tools,josharian/tools,springning/tools,jdhenke/tools,springning/tools,maddyonline/tools,nicot/tools,golang/tools,jdhenke/tools,syreclabs/go-tools,glasser/golang-tools,argandas/tools,magiccao/tools,sshetty10/tools,matryer/tools,argandas/tools,matryer/tools,sshetty10/tools,nicot/tools,cw2018/tools,dmitris/tools,josharian/tools,4ad/tools,syreclabs/go-tools,cmars/tools,golang/tools,dmitris/tools,nicot/tools,syreclabs/go-tools,springning/tools,cmars/tools,argandas/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,4ad/tools,runcom/tools