Enable table striping options

[rich-iannone]

There are settings in tab_options() that allow us to enable/disable row striping both in the table body and also in the stub. Unfortunately, these options didn't do anything. With this PR, they are now operational.

Here are a few examples. First, here's the default case:

library(gt)
library(tidyverse)

exibble %>%
  select(num, currency, row, group) %>%
  gt(groupname_col = "group", rowname_col = "row")

Here is row striping both in the table body and also in the table stub:

exibble %>%
  select(num, currency, row, group) %>%
  gt(groupname_col = "group", rowname_col = "row") %>%
  tab_options(
    row.striping.include_table_body = TRUE,
    row.striping.include_stub = TRUE
  )

One last example, where the stub has stripes and the table body does not:

exibble %>%
  select(num, currency, row, group) %>%
  gt(groupname_col = "group", rowname_col = "row") %>%
  tab_options(
    row.striping.include_table_body = FALSE,
    row.striping.include_stub = TRUE
  )

This fixes #199.