Add the 'closed' argument to the lines method.

* When 'closed' is true, an 'h' operator is included in the PDF code generated
  by the lines method, so the subpath is closed.
* The option is used in the triangle method, so it generates a closed triangle,
  with all the corners rendered with the same join style.
* @param descriptions for 'style' and 'closed' were added to the documentation
  of the lines method.
Merge pull request #112 from WarrenWeckesser/lines-closed-argument

Add the 'closed' argument to the lines method.