Feature Request: deduplicate columns/extract unique columns

[peterjc]

We can use the Unix command line tools sort and uniq to remove duplicate rows in plain text table, but I find myself wanting to do something similar with duplicated columns.

For example, after doing xsv join ... there will be at least one pair of duplicated columns (the values used for the join).

I am hoping for something like a column based version of the row based dedup command proposed on #83 (see jqnatividad/qsv#26 on the qsv fork).

I suspect I could workaround this via the proposed transpose command (see #137 / #146), which has been merged in the qsv fork.