Add commas for thousand separator

[lulunac27a]

I want to format the numeric output with commas (thousand separators). For example:
1337 is 1,337
1234567 is 1,234,567

[johnkerl]

I would love it if I could support "%'d" within Miller's fmtnum -- however in the implementation of fmtnum I leverage the Go library's fmt.Sprintf which does not accept "%'d" :(

[johnkerl]

I am working on this in issue #1490