Transform: addcol (add a column)

[rufuspollock]

Propose:

addcol [{name} [{value}]]

where name is name of column (inserted in first row) and value is optional but is value inserted in each row other than first (note if you don't have a header column you leave both out.

@rossjones suggested in #4

Feel that to add I should be able to provide a title, and either:

• A fixed value for all rows (for when I am merging CSVs from multiple sources and want to add the source)
• Code to pass to map to calculate the new cell
• A list of values to 'paste' into the new column.