[RFC] Add different line width for each edge

[lucianolorenti]

This commit changes how edges are handled. Now every edge is a different series. This is related to #39.

I have doubts about the following comment:

though care must be taken not to screw up the line_z functionality. It's not an easy thing to do :-)

I would like an example to be able to test how behaves with line_z..

[diegozea]

I'm not sure. line_z looks to be working with this PR.
@mkborregaard , what do you think?

Whit this PR:

julia> using GraphRecipes

julia> using Plots

julia> a = [ i/j for i in 1:3, j in 1:3]
3×3 Array{Float64,2}:
 0.995027   0.743106   0.703484
 0.360901   0.718034   0.416905
 0.0437366  0.0255265  0.945805

julia> graphplot(a, line_z=vec(a)')

[mkborregaard]

Nice, thanks!
@JackDevine it would be great if you could have a look at this, and merge if it doesn't interfere with any of your plans.

[JackDevine]

This looks good to me!

I wonder if an anonymous function is the best interface for this. In #39 , the OP suggested being able to pass a vector of weights. Personally, I would like graphplot to look at the adjacency matrix and work out the edge weights by itself, so that I don't have to write

graphplot(a, edgewidth=(s,d,w) -> a[s,d])

In any case, the actual visualization part looks really good to me, we can add convenience later.