iebaltab: Add `SETtest` option

[moritzpoll]

In light of journals like the AER disallowing the use of significance stars, one might want to display the difference column(s) as mean difference with its appropriate standard error to allow readers their own inference. I've implemented this. Specifying the option SETtest adds standard errors in parentheses. This option does not work in combination with PTtest. I've done a reasonable amount of testing, but of course the use of this code is as always at your own peril.

[bbdaniels]

Hi, thanks for this and it is a great idea! @luizaandrade – can we make sure this is properly incorporated into the rewrite?

[moritzpoll]

Great :) I guess I'm outing myself as a LaTeX user as I haven't adapted this to the browse or save() options yet. Glad you found it helpful! Keep up the good work everybody!

[luizaandrade]

Sorry I missed this, guys! And thank you for the addition, @moritzpoll, that's great. The new version of iebaltab is on top of my to-do list.