title: merge edgelists to a single graph
body: <p>I have three edgelists that have same nodes. I want to merge them into one graph and seperate these edges by colors and weights. I have provide small example of what I want to do:</p>
  
  <pre><code>df1
  a   b   1   blue
  b   c   0.361973313 blue
  a   d   0.343729742 blue
  df2
  a   c   0.264800107 green
  a   a   0.228507399 green
  c   d   0.22202394  green
  df3
  d   d   0.179089391 red
  d   a   0.173410831 red
  c   b   0.093636709 red
  </code></pre>
  
  <p>top dataframes are my edgelists. As you can see multiple edges and loops are free to have. A way that came to my mind to merge these edges to a single graph was to make a empty graph and then add these edges seperately, but I couldn't do it. Any idea? </p>
  
  <pre><code>g &lt;- make_empty_graph(n = 0, directed = F)
  g &lt;- g + vertices(c("a","b", "c","d"))
  
  g&lt;- g+ edges(c( "a", "b", "b", "c",
                  "a", "d"),color="blue")
  </code></pre>