Add support for vce option in Stata

[bvkrauth]

The stata rcr command accepts the cluster option but not the newer vce option.

• [x] Add vce(cluster clustvar) option
• [x] See if it's feasible to support the bootstrap and jackknife options.
• [x] Update help

Need to keep cluster option for backwards compatibility.

[bvkrauth]

Not worthwhile to support jackknife or bootstrap, users can just use the prefix version