write_xlsx directory

[ggrothendieck]

Suggest that write_xlsx allow a directory as the second argument if the first argument is a named list of data frames in which case they are all written to that directory with an extension of xlsx. That would allow

L <- list(BOD = BOD, BOD10 = 10 * BOD)
write_xlsx(L, ".")

instead of

write_xlsx(L, sprintf("./%s.xlsx", names(L)))