[R/add_border] add_border and wb_add_border

[JanMarvin]

Create borders for cell regions. This creates border styles for a specific region.

• add tests
• merge styles if possible. If cell as <xf../> node, copy the node and applyBorder and borderid. Otherwise if two or more cells share the style, they all would get the border.
• inner grid (maybe providing a single style)

[JanMarvin]

unrelated fix for showGridLines option and newly introduced rowColHeaders option.

[JanMarvin]

Any clue why the test are complaining @jmbarbone ?

[jmbarbone]

Any clue why the test are complaining @jmbarbone ?

@JanMarvin, it looks like some weird roxygen2 behavior where the the lines changes in
237a1bd were being appended to the documentation.

[JanMarvin]

Thanks, didn't see that. Maybe because of the example in the wbWorkbook part