go.talks/pkg/present: substitute tabs with 4 spaces in preformatted blocks
go.talks/pkg/present: substitute tabs with 4 spaces in preformatted blocks

Browsers tend to render tabs badly.
This is the same approach we took in godoc.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/7228062

webfd/go-zh.talks,webfd/go-zh.talks,webfd/go-zh.talks,d0f/go-zh.talks,bryanxu/go-zh.talks,bryanxu/go-zh.talks,d0f/go-zh.talks,webfd/go-zh.talks,bryanxu/go-zh.talks,d0f/go-zh.talks,d0f/go-zh.talks,bryanxu/go-zh.talks,webfd/go-zh.talks,d0f/go-zh.talks,webfd/go-zh.talks,bryanxu/go-zh.talks,bryanxu/go-zh.talks,d0f/go-zh.talks