Allow AbstractDict for edge attributes

[simonschoelly]

I noticed that edge attributes can be specified by a Dict but not an AbstractDict. Would it make sense to allow also AbstractDict? Then for example one could use DefaultDict from DataStructures.jl to specify some edge attributes.

[JackDevine]

I agree, I don't think that I had any really good reasons for choosing Dict, other than that is the data structure that I use 99% of the time. Feel free to make a pull request with this feature, otherwise I will add it to my todo list.