Update to vignettes

[jaspercooper]

closes #147

 - [x] process_tracing_designer
 - [x] factorial_designer
 - [ ] two_by_two_designer
 - [x] two_arm_attrition_designer
 - [x] binary_iv_designer
  - [x] block_cluster_two_arm.Rmd
  - [x] cluster_sampling.Rmd
  - [x] mediation_analysis.Rmd
  - [x] multi_arm.Rmd
  - [x] pretest_posttest.Rmd
  - [x] randomized_response.Rmd
  - [x] regression_discontinuity.Rmd
  - [x] simple_spillover.Rmd
  - [x] simple_two_arm.Rmd