Add a new output format "jira" to kable() - Issue #2024

[pedropark99]

Hello! This PR seeks to fulfill the request made in the issue #2024 . It adds a new output format to kable(), more specifically, a "jira" format. A simple example:

tab = head(ggplot2::mpg)
knitr::kable(tab, format = 'jira')

||manufacturer ||model || displ|| year|| cyl||trans      ||drv || cty|| hwy||fl ||class   ||
|audi         |a4    |   1.8| 1999|   4|auto(l5)   |f   |  18|  29|p  |compact |
|audi         |a4    |   1.8| 1999|   4|manual(m5) |f   |  21|  29|p  |compact |
|audi         |a4    |   2.0| 2008|   4|manual(m6) |f   |  20|  31|p  |compact |
|audi         |a4    |   2.0| 2008|   4|auto(av)   |f   |  21|  30|p  |compact |
|audi         |a4    |   2.8| 1999|   6|auto(l5)   |f   |  16|  26|p  |compact |
|audi         |a4    |   2.8| 1999|   6|manual(m5) |f   |  18|  26|p  |compact |

To achieve this new functionality, a new argument called sep.head.col in kable_mark(), and a new function called kable_jira() were added. This argument defines the separator between each column in the header of the table (i.e. the line with the column names). With format = "jira", this argument will be set to sep.head.col = "||".

If the user does not provide a value (or give a NA value) for the argument sep.head.col, it will be automatically set to the sep.col value. Because of this behaviour, in other output formats, like "pipe" or "simple", the sep.head.col argument will be equal to sep.col:

https://github.com/pedropark99/knitr/blob/2024-jira-kable/R/table.R#L393-L394

So kable_jira() basically uses kable_mark() with sep.head.col = "||" to build the output table. By adding this new sep.head.col argument to kable_mark(), we might create more flexibility to add new styles and formats in the future for kable().

Thank you for the attention!

[yihui]

Thank you so much!