Add tab for displaying godoc (at any commit, branch, PR, CL).

[dmitshur]

This is a feature request of adding a tab for displaying the godoc of a Go package.

The motivation for having this (despite godoc.org being available and serving as an excellent source of Go documentation) is to be able to leverage the ability of gotools.org of displaying Go packages at any commit, branch, PR, CL, etc.

This will make it possible to preview what the godoc of a Go package will look like after a PR/CL is merged, before merging that PR/CL. This ability can be desirable sometimes. (Also see here for motivation.)

Implementation

This needs to be implemented in a way that won't require too much maintenance, and has high likelihood of producing correct godoc results. Otherwise, this feature will cause more harm than good. The gtdo project provides a build.Package at any commit/branch/PR/CL, all that needs to be implemented is producing godoc HTML/CSS/JS for said build.Package and serving it.