Apply column formats to slider labels

[mikmart]

Fixes #247.

The format* functions append new columnDefs entries with customized renderer functions to apply column formatting. The idea in this PR is to fetch those same renderer functions to apply to filter slider labels.

DT::datatable(
  tibble::tibble(
    goal_completion = round(runif(10), 2),
    bonus = goal_completion * 15:24 * 1000,
    payday = Sys.Date() + 1:10
  ),
  filter = "top"
) |>
  DT::formatPercentage(1) |>
  DT::formatCurrency(2) |>
  DT::formatDate(3)

[mikmart]

Thank you for the kind words!

I think with these changes the previously special-cased date formatting can also be simplified. I'm planning to have a stab at refactoring that in another PR.