[WIP] Add support for Julia

[findmyway]

Hi guys,

I would like to add a Julia wrapper for VisualDL.

Previously I've written a wrapper for VisualDL by leveraging PyCall.jl. Obviously there's a little overhead.

Recently I have spent some time on how to use the C++ SDK of VisualDL by using CxxWrap.jl. And it works fine.

This PR should give you a glimpse of the up-coming changes to support Julia. Before updating vdljl.cc to support more datatypes (histogram, text, voice...), I would like to hear your thoughts/suggestions.

Cheers!

[nepeplwu]

I wonder if vdl.jl is a demo or will be a julia sdk, if it is a demo, move it to demo dir is more reasonable

[findmyway]

@nepeplwu
Thanks for your review!
Yes, it's just a quick demo. And I'll remove it first and provide a more detailed demo later.

The whole logic is that:

1. Add CxxWrap.jl as a dependency in VisualDL.jl
2. In the building state of VisualDL.jl, the source code of VisualDL will be downloaded and compiled to a .so file. (JlCxx_DIR specified here)

Once this PR get merged. I'll update the README file to describe how to use it in julia (which is super easy!). Also if you agree, I can transfer VisualDL.jl to this org for better maintainance.

[nepeplwu]

@findmyway , Thank you for your contribution to VisualDL, this PR looks good to me. I recommend updating the README doc(which is in docs/release) and describe the entire step about building a julia-enable package(such as how to install JlCxx)

[findmyway]

Thanks

Sure, once JuliaLang/MbedTLS.jl#193 get fixed, I'll make another PR to document the whole pipeline and provide some interesting demos.