Theoretical distributions to visualize and a few wrapper functions

• Switched to !! instead of UQ() since UQ() is deprecated in
  {rlang} 0.2.0
• Added many new files: CONDUCT.md, CONTRIBUTING.md, and TO-DO.md
• Updated README file with more development information
• Added wrapper functions t_test() and chisq_test() that use a
  formula interface and provide an intuitive wrapper to t.test() and
  chisq.test()
• Created stat = "z" and stat = "t" options
• Added many new arguments to visualize() to prescribe colors to shade and
  use for observed statistics and theoretical density curves
• Added check so that a bar graph created with visualize() if number of
  unique values for generated statistics is small
• Added shading for method = "theoretical"
• Implemented shading for simulation methods w/o a traditional distribution
  • Use percentiles to determine two-tailed shading
• Changed method = "randomization" to method = "simulation"
• Added warning when theoretical distribution is used that
  assumptions should be checked
• Added theoretical distributions to visualize() alone and as overlay with
  current implementations being
  • Two sample t
  • ANOVA F
  • One proportion z
  • Two proportion z
  • Chi-square test of independence
  • Chi-square Goodness of Fit test
  • Standardized slope (t)