Include option to hide all column labels

[rich-iannone]

Sometimes it can be very useful not to have column labels at all. This PR makes that possible by providing the column_labels.hide argument in tab_options(). Here is a working example:

# Create a few Markdown-based
# text snippets
text_1a <- "
 ### This is Markdown.

 Markdown’s syntax is comprised entirely of
 punctuation characters, which punctuation
 characters have been carefully chosen so as
 to look like what they mean... assuming
 you’ve ever used email.
 "

text_1b <- "
 Info on Markdown syntax can be found
 [here](https://daringfireball.net/projects/markdown/).
 "

text_1c <- "<img src=\"https://i.imgur.com/3RqWtfL.gif\" width=\"200\">"

text_2a <- "
 The **gt** package has these datasets:

  - `countrypops`
  - `sza`
  - `gtcars`
  - `sp500`
  - `pizzaplace`
  - `exibble`
 "

text_2b <- "
 There's a quick reference [here](https://commonmark.org/help/).
 "

text_2c <- "
 Yes, gifs should work.
 "

# Arrange the text snippets as a tibble
# using the `dplyr::tribble()` function;
# then, create a gt table and format
# all columns with `fmt_markdown()`
dplyr::tribble(
  ~Markdown, ~md,
  text_1a,   text_2a,
  text_1b,   text_2b,
  text_1c,   text_2c,
) %>%
  gt() %>%
  fmt_markdown(columns = TRUE) %>%
  tab_options(
    table.width = px(500),
    column_labels.hide = TRUE
  )

Closes #225.

[jcheng5]

Missing tests... other than that, LGTM. Don't forget to cardify and send through QA.