Fix/917 docx

[r2evans]

fix #917

This PR supports inline use,

kableExtra::kbl(mtcars[1:8, 1:4], booktabs = TRUE, linesep = "", format = "docx") |>
  kableExtra::kable_styling(latex_options = "striped", stripe_index = c(1,2, 5:6)) |>
  save_kable("foo.png")
# file:////private/var/folders/3f/0hlmq7rx34ddjqtlc1lqbxl80000gn/T/RtmpvBKq7w/fileaa143902958.html screenshot completed
# [1] "foo.png"

as well as Rmd rendering

---
title: "Use kable for DOCX"
output: rmarkdown::word_document
---

```{r}
kableExtra::kbl(mtcars[1:8, 1:4], booktabs = TRUE, linesep = "", format = "docx") |>
  kableExtra::kable_styling(latex_options = "striped", stripe_index = c(1,2, 5:6))
```

rmarkdown::render("vignettes/use_kable_in_docx.Rmd", output_file="foo.docx")

Caveat: I did a global search for format == "html" and shifted to %in% c(..), I do not know if that might not be desired in all cases. Also, the first commit here centralized the warning to reduce how many places to go for updating it (it made sense, I can pull it back if you don't like it).