Semver Support 

Github endpoints should support semver ranges (there are use cases where commit-ish are inadequate when using github as a temporary package manager). npm supports using semver (similar to commit-ish) with github repositories with the syntax:
```
"<repo name>":"github:<id>/<repo>#semver:<semver range>
``` 

Please add this support. I am guessing that the code can be ripped off from npm.