Let exporter inherit from SlidesExporter to expose more resources

[mkcor]

Hello,

With this change, we are paving the way for the reveal template. Without it, using the reveal template yields a 500 Internal Server Error.

Steps to reproduce:

• Copy directory share/jupyter/voila/template/reveal from https://github.com/QuantStack/voila/pull/54/files into PREFIX/share/jupyter/voila/templates/;
• Get notebooks/rise_example.ipynb from https://github.com/QuantStack/voila/pull/54/files and (from where the notebook lives) run

voila rise_example.ipynb --template=reveal

We now have

ipdb> VoilaExporter.mro()                                                                                                                  
[<class 'voila.exporter.VoilaExporter'>, <class 'nbconvert.exporters.slides.SlidesExporter'>, <class 'nbconvert.exporters.html.HTMLExporter'>, <class 'nbconvert.exporters.templateexporter.TemplateExporter'>, <class 'nbconvert.exporters.exporter.Exporter'>, <class 'traitlets.config.configurable.LoggingConfigurable'>, <class 'traitlets.config.configurable.Configurable'>, <class 'traitlets.traitlets.HasTraits'>, <class 'traitlets.traitlets.HasDescriptors'>, <class 'object'>]

instead of

ipdb> VoilaExporter.mro()                                                                                                                  
[<class 'voila.exporter.VoilaExporter'>, <class 'nbconvert.exporters.html.HTMLExporter'>, <class 'nbconvert.exporters.templateexporter.TemplateExporter'>, <class 'nbconvert.exporters.exporter.Exporter'>, <class 'traitlets.config.configurable.LoggingConfigurable'>, <class 'traitlets.config.configurable.Configurable'>, <class 'traitlets.traitlets.HasTraits'>, <class 'traitlets.traitlets.HasDescriptors'>, <class 'object'>]

/cc @maartenbreddels

[mkcor]

We won't have the voila exporter inherit from SlidesExporter (see discussion on Gitter).
Using this branch just for experimentation from now on...