Nbsite

- Added unit test infrastructure
- Added `--skip` argument to `generate-rst` to support skipping notebooks.
- Adding `.nojekyll` file now by default as part of `build`
- Allow configuration of the notebook download link placement with `--nblink` on `generate-rst`

Full list of changes since the previous release: https://github.com/pyviz/nbsite/compare/v0.4.7...v0.4.8

* Fix build command's exit code (85)

* Still problems with nbconvert 5.4, so pin <5.4 for now (see 84 for more details)

* Fix error removing sphinx file not present in build dir with sphinx>=1.8

* Support sphinx 1.8
* Support nbformat 5.4