Add prehead() option to iebaltab

[avitabilea]

Hi Team,

Thanks for developing and maintaining this great package. I think it would be great if the iebaltab function came with a prehead() option, similar to estout. This would allow users to create more flexible balance tables by setting up the tabular or tabluarx environments in LaTeX. For example, it is not currently possible to create the attached Duflo et al. (2015) balance table with separate panels using the tool as it stands.

[avitabilea]

Closing this issue as I realize it's the same as this