Add p_value and conf_int functions and observed stat shortcut with specify() %>% calculate()

• Added conf_int logical argument and conf_level argument to t_test()
• Switched shade_color argument in visualize() to be pvalue_fill instead
  since fill color for confidence intervals is also added now
• Shading for Confidence Intervals in visualize()
  • Green is default color for CI and red for p-values
  • direction = "between" to get the green shading
  • Currently working only for simulation-based methods
• Implemented conf_int() function for computing confidence interval provided a simulation-based method with a stat variable
  • get_ci() and get_confidence_interval() are aliases for conf_int()
  • Converted longer confidence interval calculation code in vignettes to use get_ci() instead
• Implemented p_value() function for computing p-value provided a simulation-based method with a stat variable
  • get_pvalue() is an alias for p_value()
  • Converted longer p-value calculation code in vignettes to use get_pvalue() instead
• Implemented Chi-square Goodness of Fit observed stat depending on params being set in hypothesize with specify() %>% calculate() shortcut
• Removed "standardized" slope $t$ since its formula is different than "standardized" correlation and there is no way currently to give one over the other
• Implemented correlation with bootstrap CI and permutation hypothesis test
• Filled the type argument automatically in generate() based
  on specify() and hypothesize()
  • Added message if type is given differently than expected
• Implemented specify() %>% calculate() for getting observed
  statistics.
  • visualize() works with either a 1x1 data frame or a vector
    for its obs_stat argument
  • Got stat = "t" working
• Refactored calculate() into smaller functions to reduce complexity
• Produced error if mu is given in hypothesize() but stat = "median"
  is provided in calculate() and other similar mis-specifications
• Tweaked chisq_stat() and t_stat() to match with specify() %>% calculate() framework
  • Both work in the one sample and two sample cases by providing formula
  • Added order argument to t_stat()
• Added implementation of one sample t_test() by passing in the mu argument to t.test
  from hypothesize()
• Tweaked pkgdown page to include ToDo's using {dplyr} example