Use vl-convert for offline html export

[jonmmease]

This PR takes advantage of HTML bundling support in vl-convert 1.0 to replace the dependency on altair_viewer when calling chart.save('chart.html', inline=True) to create an HTML file with inline JavaScript dependencies.

The vl-convert javascript_bundle() function returns a bundle with Vega, Vega-Lite, and Vega-Embed which can be included in a <script> tag. This function actually performs bundling and minification and takes 1-2s the first time it's called. The result is cached and subsequent calls are much faster.

I had to update the vl-convert minimum dependency to 1.0.1 because I discovered a bundling bug during integration.

[mattijn]

Thanks @jonmmease!
Would it be possible to update the allowed arguments and docstrings here, for chart.save(), using the complete arguments and docstrings from here (save function within altair.utils.save)?

Maybe this, for chart.to_html(), is incomplete too?

I noticed this when there was no insight on the new argument in the tool tip when trying to save a chart:

[jonmmease]

Thanks for the review @mattijn, this is ready for another look

[mattijn]

Looks good now! This is great @jonmmease!