Add padding options for heading and column labels

[rich-iannone]

This PR adds two arguments to tab_options(): column_labels.padding and heading.padding. Several options already exist for easily changing the vertical padding of certain locations, so, these additions make it possible to fully alter to the padding from top to bottom (without resorting to CSS).

Here's some examples of different padding levels applied to the same table (where the padding is 0, 5, and 10px):

library(gt)

padding_px <- x

mtcars[1:5, ] %>%
  gt(rownames_to_stub = TRUE) %>% 
  tab_header(title = "The mtcars Dataset", subtitle = "What a great dataset this is") %>% 
  tab_spanner(label = "performance", columns = c(disp, hp, drat)) %>%
  tab_footnote(footnote = "A table footnote", locations = cells_title("title")) %>%
  tab_source_note(source_note = "Dataset is generally available in R") %>%
  tab_options(
    data_row.padding = px(padding_px),
    column_labels.padding = px(padding_px),
    heading.padding = px(padding_px),
    footnotes.padding = px(padding_px),
    source_notes.padding = px(padding_px)
  )

Here's the table without the tab_options() call (default padding):

Fixes: #770