Enhance github downloader logic

[efrecon]

Some projects, e.g. bashtop, see #22, do not make releases through the gitthub UI (or the command-line equivalents such as gh or hub), but simply through pushing annotated tags. For such projects, the release API will return an empty JSON array, meaning the assets to get is a copy of the source code. These are pointed at by the tags api and techniques similar to what is used for releases can be used to get to the binary. In those cases, the implementation should perhaps account for a build step, e.g. running make in some form.