Spawn a subshell in the directory of the installed package specified.
If a command is specified, then it is run in the subshell, which then
Note that the package is not automatically rebuilt afterwards, so be
sure to use npm rebuild <pkg> if you make any changes.
The first element in the args parameter must be a package name. After that is the optional command, which can be any number of strings. All of the strings will be combined into one, space-delimited command.