Validate single numeric value for alpha argument

[juliasilge]

This PR closes #180 by validating that the alpha argument for the three bootstrap CI functions is a single numeric value.

library(tidyverse)
library(rsample)
library(broom)

lm_est <- function(split, ...) {
  lm(mpg ~ disp + hp, data = analysis(split)) %>%
    tidy()
}

set.seed(52156)

car_rs <-
  bootstraps(mtcars, 200, apparent = TRUE) %>%
  mutate(results = map(splits, lm_est))

int_pctl(car_rs, results, alpha = c(0.05, 0.2))
#> Error: `alpha` must be a single numeric value.
#> Backtrace:
#>     █
#>  1. └─rsample::int_pctl(car_rs, results, alpha = c(0.05, 0.2))
int_t(car_rs, results, alpha = "potato")
#> Error: `alpha` must be a single numeric value.
#> Backtrace:
#>     █
#>  1. └─rsample::int_t(car_rs, results, alpha = "potato")
int_bca(car_rs, results, alpha = 1:4, .fn = lm_est)
#> Error: `alpha` must be a single numeric value.
#> Backtrace:
#>     █
#>  1. └─rsample::int_bca(car_rs, results, alpha = 1:4, .fn = lm_est)

int_bca(car_rs, results, alpha = 0.2, .fn = lm_est)
#> Warning: Recommend at least 1000 non-missing bootstrap resamples for terms:
#> `(Intercept)`, `disp`, `hp`.
#> # A tibble: 3 x 6
#>   term         .lower .estimate  .upper .alpha .method
#>   <chr>         <dbl>     <dbl>   <dbl>  <dbl> <chr>  
#> 1 (Intercept) 28.5      30.9    32.6       0.2 BCa    
#> 2 disp        -0.0425   -0.0299 -0.0221    0.2 BCa    
#> 3 hp          -0.0426   -0.0270 -0.0149    0.2 BCa

^{Created on 2020-09-14 by the reprex package (v0.3.0.9001)}

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