Make mail background color configurable

[salim-b]

Currently, the mail background color is hardcoded to #f6f6f6. I'd like to change that to #f2f1f0 which is the default GUI grey in Ubuntu Linux. To do so, I currently modify the object created by blastula::compose_email() as follows:

blastula::compose_email() %>%
  purrr::list_modify(html_str =
                       .$html_str %>%
                       stringr::str_replace_all(pattern = "(?i)#f6f6f6",
                                                replacement = "#f2f1f0"))

I think it would be cool to have a param background_color for blastula::compose_email() to directly specify the color to use. Would you welcome a PR implementing this?

[rich-iannone]

This would be a great PR, thanks!

[algo-se]

this type of customization would be great!

btw, I think the op hack does not work anymore... :(