Add support for direct building of a URL

This allows for things like: (to build test a PR branch directly)
```console
$ ./build.sh https://raw.githubusercontent.com/infosiftr/stackbrew/redis/library/redis
```

Or: (to build test just one tag of a PR branch)
```console
$ ./build.sh https://raw.githubusercontent.com/infosiftr/stackbrew/redis/library/redis:2.6
```