make Juno dependency conditional

[tpapp]

Because of a dependency via Juno, using Flux on Julia#master currently suffers from JunoLab/Media.jl#19.

I wonder if dependency on Juno could be made conditional, via Requires. Looking at the code, the only difficulty I see with this is the @progress macro in training.

[MikeInnes]

We should probably just fix that issue, rather than introducing something more fragile into Flux.

[pfitzseb]

Could also switch to the

@logmsg -1 "making progress" progress=0.5 

API.

[tpapp]

I am making a PR right now using the @logmsg API.

[Roger-luo]

I was wondering if we support conditional dependencies in Pkg would help. Like rust, users can choose enhancement like GPU, better Juno printing etc.while installing the package.