Github loading

Add support for loading modules from `github`

Following syntax should be supported

```sh
import "github/hzlmn/build/test.sh"
```
Possible actions can be:
 - fetching content from `github.com` for `hzlmn` in repo `build`
 - if filename specified fetch it, if not look for `<repo_name>.sh` in this case `build.sh` and load it