tidygraph 1.2.0

• graph description now recognise undirected trees
• Added pkgdown site at https://tidygraph.data-imaginist.com
• Prepare tidygraph for dplyr 1.0.0 (#118 and #119)
• Add possibility of controlling which column in nodes are used for matching
  if the to and from columns in edges are character vectors during
  construction (#89)
• bind_graph() now accepts a list of graphs as its first argument (#88)
• Add graph_modularity() for calculating modularity contingent on a node
  grouping (#97)
• Edge weights are now handled more consistently to avoid igraph using a
  possible weight edge attribute. weights = NULL will always mean that no
  edge weight is used (#106).
• Neighborhood graph in map_local() and siblings will now contain a
  .central_node node attribute that will identify the node from which the
  local graph has been calculated (#107)