Feature request: Inherit tab_spanner from name of list of tables

[szimmer]

Is your feature request related to a problem? Please describe.
A named list is useful in making sure things are in order. It could be nice that when using tbl_merge() that the tab_spanner defaults to those names.

Describe the solution you'd like

library(survival)
library(gtsummary)
t1 <-
  glm(response ~ trt + grade + age, trial, family = binomial) %>%
  tbl_regression(exponentiate = TRUE)
t2 <-
  coxph(Surv(ttdeath, death) ~ trt + grade + age, trial) %>%
  tbl_regression(exponentiate = TRUE)

table_list <- list("**Tumor Response**"=t1, "**Time to Death**"=t2)

tbl_merge(
  tbls = table_list,
  tab_spanner = names(table_list)
)

This is what it would look like but you wouldn't need to explicitly specify the names. I would find this useful for something like this:

tbl_merge(
  tbls = list("**Tumor Response**"=t1, "**Time to Death**"=t2)
)

Additional context
This ensures the ordering is correct and is a slick default if names are present.

[ddsjoberg]

Hey hey @szimmer !

Sure, sounds like a good update. Can you submit a PR?

1. Change the default value to tab_spanner = ifelse(is_named(tbls), names(tbls), paste0("**Table ", seq_along(tbls), "**")).
2. Add unit tests in test-tbl_merge.R to ensure update works properly.
3. Update the documentation for the argument.