Add '__ignore_root' tag option to add_external_plugin

Currently if a repo such as https://github.com/garyburd/redigo is added to `plugin_loader.cmake`, the build will fail with `no buildable go files in github.com/garyburd/redigo`.

This tag creates the option to ignore the root folder creating the list of imported plugins during the build process.

Usage:
```add_external_plugin(git https://github.com/garyburd/redigo 204566ad8c530fe658a939ff8d1857ad6f6012d2 redis __ignore_root)```