Improve the stub so that it can be formatted

[rich-iannone]

These changes make it possible to format the stub with fmt_*() functions. This also adds the stub() select helper function.

The following code uses the stub() helper in a few functions. Notably, fmt_markdown() now performs Markdown translation in a stub column, cols_align() now works on the stub column, and the width of the stub column is now easier to control with stub() on the LHS of ~.

library(gt)

dplyr::tibble(
  stub = c("*one*", "**two**", "three"),
  vals = 3:1
) %>%
  gt(rowname_col = "stub") %>%
  fmt_markdown(columns = stub()) %>%
  cols_align(
    align = "center",
    columns = everything()
  ) %>%
  cols_width(
    stub() ~ px(500),
    everything() ~ px(200)
  )

Fixes: #747

[cscheid]

lgtm