`standalone` autoexecutes the entry module

Hopefully half-fixes substack/node-browserify#430 (--debug and --standalone are broken when used together) by moving the auto-execution of the entry module from browserify to browser-pack.

Unfortunate, but no clean way to insert the autoexecution from browserify: the source map is emitted from within browserpack, and the auto-exec needs to be inserted between the end of browser-pack stuff and the source map.

Depends on a forthcoming PR in node-browserify.