Modify to_dot

[clclcocoro]

Thank you for creating the amazing library.

I noticed that to_dot() function did not print the edge attributes in Graphs.graph(ExVertex[], ExEdge{ExVertex}[], is_directed=false). I modified the corresponding line in src/dot.jl and added test codes to test/dot2.jl.

Would you please review these changes?

[mschauer]

to_dot shout also allow for preamble blocks as in

    graph [bgcolor=black]
    node [
        colorscheme="spectral10"
        shape=point
        ];
    edge [
        colorscheme="spectral10"
        ];

currently only isolated attributes=value pairs are allowed.

[clclcocoro]

I understood.