Pass locale info through to vl-convert, default to display embed options when not set

[jonmmease]

This PR makes three changes related to locale handling:

1. the formatLocale and timeFormatLocale embed options are passed through to vl-convert for static image export
2. vl-convert is used to allow formatLocale and timeFormatLocale to be set using the locale name (e.g. it-IT) rather than only by setting the full locale dictionary.
3. Chart saving defaults to using the Vega embed options set with alt.renderers.set_embed_options (if any). This way you can call alt.renderers.set_embed_options once and all of the charts will be displayed and saved using the provided locale configuration.

Still needs documentation, but I haven't decided whether to tackle that in this PR or not. If folks have ideas for where best to document this functionality let me know!

cc @MarcoBaroncini

[mattijn]

Not sure if this is something to add to this PR, but still want to mention that a while ago I wished there was reset_embed_options() as I had changed the defaults but didn't know how to get back to the default...

[jonmmease]

Yeah, it seems like set_embed_options persists across kernel restarts. Do you know how that works?

[mattijn]

These comments from @domoritz around here: vega/vega-embed#170 (comment) might give more insight:

I'm somewhat hesitant to add this to Vega-Embed since formatting is a global property and not set per view.

@jheer do you have a suggestion for how we should handle formatting in Vega-Embed and Altair/Jupyter?

And

Great, this API should work. One issue I see now is that we set the locale globally. I wonder whether people would think that the locale is set per spec if it’s passed as an argument to embed when in fact it’s a global setting?

[MarcoBaroncini]

Still needs documentation, but I haven't decided whether to tackle that in this PR or not. If folks have ideas for where best to document this functionality let me know!

I suggest to document this in Customizing Renderers chapter, maybe in a dedicated subchapter? I usually add documentation via different PR, but to me it makes no difference.

What should we add? I guess how to set locale for visualization via set_embed_options , and a section dedicated to saving via chart.save().

[jonmmease]

@mattijn @MarcoBaroncini I added a new documentation section under "Customizing Visualizations". See if the description here makes sense. Note that I used a static SVG for the localization example chart output because sphinx_altair doesn't support localization yet, and even if it did it might interfere with the reset of the documentation.

I also added a changelog entry.

I think this PR is ready to go now!

[mattijn]

Thanks @jonmmease, I've added a note how to set the default and changed javascript style variable names into the equivalent python style. Once the tests pass, we can merge this.

[jonmmease]

Thanks @mattijn!