wk 0.5.0

• Fixed bugs relating to the behaviour of wk classes as
  vectors (#64, #65, #67, #70).
• crc() objects are now correctly exported as polygons
  with a closed loop (#66, #70).
• Added wk_vertices() and wk_coords() to extract individual
  coordinate values from geometries with optional identifying
  information. For advanced users, the wk_vertex_filter()
  can be used as part of a pipeline to export coordinates
  as point geometries to another handler (#69, #71).
• Added wk_flatten() to extract geometries from collections.
  For advanced users, the wk_flatten_filter() can be used as
  part of a pipeline (#75, #78).
• options("max.print") is now respected by all vector classes
  (#72, #74).
• Moved implementation of plot methods from wkutils to wk to
  simplify the dependency structure of both packages (#80, #76).
• Added wk_polygon(), wk_linestring(), and wk_collection()
  to construct polygons, lines, and collections. For advanced
  users, wk_polygon_filter(), wk_linestring_filter(), and
  wk_collection_filter() can be used as part of a pipeline
  (#77, #84).
• Added a C-level transform struct that can be used to simplify
  the the common pattern of transforming coordinates. These
  structs can be created by other packages; however, the
  wk_trans_affine() and wk_trans_set() transforms are
  also built using this feature. These are run using the
  new wk_transform() function and power the new
  wk_set_z(), wk_set_m(), wk_drop_z(), wk_drop_m(),
  functions (#87, #88, #89).