Vega support

[mcmcgrath13]

This PR proposes a few changes to increase support for Vega specifications:

• Update the MIME type for VGSpecs to "application/vnd.vega.v4+json", which is the current standard for JupyterLab (have to install an extension for the current v3 to show interactively)
• Create an AbstractVegaSpec type to simplify some mostly duplicate functions
• Add a section to the docs on using Vega and how to use it in a function (I had some trouble getting this to work and it feels slightly hack-y - @fredo-dedup do you have a better way to do this?)

I noticed the examples folder seems to use an outdated syntax - can that be removed as well?

This PR shouldn't be merged until the corresponding IJulia PR is merged so they are ready to support the v4 mime type.

I expect this will need to be updated shortly as Vega recently released v5.

[codecov-io]

Codecov Report

Merging #153 into master will decrease coverage by 1.05%.
The diff coverage is 41.66%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   69.28%   68.23%   -1.06%     
==========================================
  Files          17       17              
  Lines         661      617      -44     
==========================================
- Hits          458      421      -37     
+ Misses        203      196       -7

Impacted Files	Coverage Δ
src/VegaLite.jl	100% <ø> (ø)	⬆️
src/vgspec.jl	100% <ø> (ø)	⬆️
src/rendering/io.jl	100% <ø> (+3.22%)	⬆️
src/rendering/fileio.jl	25% <0%> (ø)	⬆️
src/vlspec.jl	100% <100%> (ø)	⬆️
src/rendering/show.jl	86.27% <40%> (+0.71%)	⬆️

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[coveralls]

Coverage increased (+0.2%) to 68.947% when pulling 746e8f6 on mcmcgrath13:vega_support into dd93463 on fredo-dedup:master.

[mcmcgrath13]

Travis is failing on mac due to an issue with the Cairo build - I need to dig into this

[davidanthoff]

Awesome, this is fantastic and in really good shape!! Only one comment, and then we can merge this :)

[davidanthoff]

Travis is failing on mac due to an issue with the Cairo build - I need to dig into this

That unfortunately happens regularly... It is really frustrating, the whole BinDeps.jl binary provider story that Cairo.jl utilizes is just not reliable and seems to break a lot... I think the only proper solution is that someone creates a BinaryBuilder.jl style solution for Cairo.jl and Rsvg.jl. But that implies Gtk.jl also needs to be ported, and at that point it is a mega project...

In any case, that failure is clearly not related to your PR, so I think we can merge things here independently of the Cairo.jl situation.

[mcmcgrath13]

@davidanthoff that's good to know about Cairo... (kind of - it's always nicer when things just work). I've gitignored the project.toml and when this is merged I'll do another PR with just the Project.toml and the updated travis script so it can be merged whenever attobot is ready.

On the IJulia side, I've updated that PR to preemptively include the mime types for vega v5 and vegalite v3 so we should be able to switch those over whenever they're supported by jupyter.

[davidanthoff]

Super, thanks a lot!