* Add pkger source driver support
As go-bindata has been abandoned [1] there are open requests, #116, for
alternative sources with similar functionality. The Buffalo project [2]
created packr and recently pkger [3] was announced [4] with the
intention to supersede packr.
This change adds support for using pkger as a source.
The implementation relies on httpfs.PartialDriver for pretty much all
functionality.
[1] https://github.com/jteeuwen/go-bindata/issues/5
[2] https://gobuffalo.io/
[3] https://github.com/markbates/pkger
[4] https://blog.gobuffalo.io/introducing-pkger-static-file-embedding-in-go-1ce76dc79c65
* pkger: rename Instance to Pkger
* pkger: make WithInstance accept *Pkger
* pkger: refactor and add access to global pkging.Pkger instance
* pkger: fix typo and cleanup debug logging
