Updates to cols_merge()

[ddsjoberg]

Proposed updates to cols_merge()

• Add cols_merge(rows=) argument. Only columns and rows selected will merged. But we will need some sort of mechanism for handle default values
• Add cols_merge(pattern_missing=) argument. This would specify what the default value of the cell should be when the cell is not merged with others. A reasonable default value here would be an empty string (like how <<>> handles missing cells. We also would like to be able to add something like cols_merge(pattern = "{1} ({2})", rows = !is.na(ci), pattern_missing="{1}")

[rich-iannone]

Thanks for filing this issue Dan! These proposed update both make a lot of sense and this is all something I could get to working on quickly.