Allow custom formats, NA values in key columns

[edelarua]

Closes #157

[github-actions]

Code Coverage Summary

Filename                 Stmts    Miss  Cover    Missing
---------------------  -------  ------  -------  ---------------------------------------------------------
R/paginate_listing.R        40       1  97.50%   99
R/rlistings_methods.R      101      14  86.14%   39, 54, 58, 137-140, 143, 227-233
R/rlistings.R              159      20  87.42%   156-157, 160-161, 200-202, 366-369, 373-376, 405, 409-412
TOTAL                      300      35  88.33%

Diff against main

Filename         Stmts    Miss  Cover
-------------  -------  ------  -------
R/rlistings.R       +3       0  +0.24%
TOTAL               +3       0  +0.12%

Results for commit: 4e4fa2f

Minimum allowed coverage is 80%

♻️ This comment has been updated with latest results

[github-actions]

Unit Tests Summary

  1 files    4 suites   5s ⏱️
27 tests 21 ✔️ 6 💤 0 ❌
71 runs  64 ✔️ 7 💤 0 ❌

Results for commit 52ecfd1.

♻️ This comment has been updated with latest results.

[Melkiades]

I think all is very good. Only I found a case that I think we should trim, when a row is only NAs.

data(mtcars)
mtcars[2, ]
lsting <- as_listing(
    mtcars,
    key_cols = c("gear", "carb"),
    disp_cols = "qsec"
)

produces something that ends like:

        8     14.6 
 NA     NA     NA  

I think the lastline is not needed