Update chi squared tests

infer 0.5.0

Breaking changes

• shade_confidence_interval() now plots vertical lines starting from zero (previously - from the bottom of a plot) (#234).
• shade_p_value() now uses "area under the curve" approach to shading (#229).

Other

• Updated chisq_test() to take arguments in a response/explanatory format, perform goodness of fit tests, and default to the approximation approach (#241).
• Updated chisq_stat() to do goodness of fit (#241).
• Make interface to hypothesize() clearer by adding the options for the point null parameters to the function signature (#242).
• Manage infer class more systematically (#219).
• Use vdiffr for plot testing (#221).