Issue 58 compare given columns

[zkamvar]

this will fix #58:

library("linelist")
ichar <- iris
levels(ichar$Species) <- c("hickory", "dickory", "setosa")
compare_data(iris, ichar, columns = -5)
#> 
#>  /// Comparisons of data content // 
#> 
#> 
#>  // Comparison of dimensions /
#> Same number of rows and columns
#> 
#>  // Comparison of variable names /
#> 
#> Same variable names, in the same order
#> 
#>  // Comparison of variable classes /
#> 
#> Same variable classes
#> 
#>  // Comparison of values in categorical variables /
#> 
#> Same values for categorical variables

^{Created on 2019-04-10 by the reprex package (v0.2.1)}