emacs: add versatile "emacsWrapper"

"emacsWrapper" replaces emacsWithPackages. In addition to "packagesFun",
emacsWrapper has an optional variable called "execStart". execStart can
be used to append elisp to the default site-start.el script. This is
useful for providing a way to load a user's .emacs.d/init.el
file. "emacsWithPackages" is implemented with emacsWrapper for
convenience and compatability.