Add vl-convert as dependecy for conda vegafusion package

[joelostblom]

When I run the conda installation command from the docs conda install -c conda-forge vegafusion vegafusion-python-embed vegafusion-jupyter and then try to use vegafusion, I see this error:

ImportError: The vl-convert Vega-Lite compiler requires the vl-convert-python package

Installing vl-convert manually via conda fixes the issue (but I do run into #311 afterwards). I think it would be nice if there was one single conda package (vegafusion or vegafusion-python or vegafusion-altair) which installed everything required to use vegafusion with altair/python

[jonmmease]

Thanks for the suggestion @joelostblom, the reason vl-convert isn't a dependency right now is that it's not required to use VegaFusionWidget, but since vl-convert-python is available for every architecture that vegafusion is built for, I do think it would make sense to add it as a dependency.