implement path_effects for Line2D objects
modify patheffect_demo.py to demonstrate patheffects for Line2D objects
initial implementation of patheffect for collections
patheffect.draw_path_collection work with mixed renderer.
add tests for patheffects
Add a sketch path filter.
Fix oops in _path.cpp
Add xkcd() function to set up a number of parameters to create plots that look like xkcd.  Add an example, and a what's new.  Expose path effects as an rcParam so that they can be set globally.
Credit basis for plots
Change behavior of `font.*` rcParams so they take effect only on to-be-created text objects
Miscellaneous fixes mentioned in the PR
Provide xkcd as a context manager
Add credit to figures themselves
Merge pull request #1329 from mdboom/wiggles

Add a "sketch" path filter for XKCD style graphics.