gopages: exclude folders from index

[corani]

In my repository I'm (1) vendoring my dependencies in /vendor and (2) vendoring various tools such as code generators in /tools/vendor. These all end up in the index when running gopages (though no files are generated in dist/pkg, so they end up with a 404). It would be nice if the folders could be excluded somehow, e.g. via a command-line argument.

[JohnStarich]

Hmm that's an interesting one. Perhaps this kind of filtering is done internally to normal godoc with the package list (go list ./...).

I'm open to a PR with a test to reproduce and a bug fix. (If no one picks it up for a while, I may give it a try.)