Allow getproperty and setproperty! syntax for vertex labels

[CameronBieganek]

It would be cool if the following syntax were supported:

g[:a].color = "blue"

g[:a, :b].size = 2

[bramtayl]

This might be nice. I think I would like to maintain the consistency with the Dict interface. If this goes through in Base, I would absolutely be on board JuliaLang/julia#25750

[bramtayl]

Then again, if JuliaLang/julia#25750 goes through, this will happen automatically. What would be nice to have though is getproperty and setproperty for vertex labels, so I'm renaming the issue. Let me know if you disagree.

[CameronBieganek]

Then again, if JuliaLang/julia#25750 goes through, this will happen automatically.

That's technically a breaking change, so it probably won't go through until 2.0. See here.

[gdalle]

Closing since we won't do it unless Julia does it