go.talks/pkg/present: fix named highlights
go.talks/pkg/present: fix named highlights

Previously it wasn't stripping "// HLfoo" comments from lines.

R=campoy, campoy
CC=golang-dev
https://codereview.appspot.com/7193054

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