talksapp: Allow specifying branch for github url

[smasher164]

Currently, non-master branches are not recognized on go-talks.appspot.com. gosrc/github.go hardcodes the master branch for the directory path. https://github.com/golang/gddo/blob/master/gosrc/github.go#L197

rawBase, err := url.Parse(expand("https://raw.github.com/{owner}/{repo}/master{dir}/", match))

Related issue: #464 (for docs)

This also leads to issues where the .slide file is presented but the assets are not retrieved. See this slide.
The ability to specify a specific branch would greatly benefit my project.

[lukegb]

https://go-review.googlesource.com/c/42653/ solves the case where the assets for a presentation are stored alongside the presentation in a non-master default branch, but it doesn't try to support specifying a particular branch to retrieve files from.