npm-install - install a package programmatically
npm.commands.install([where,] packages, callback)
This acts much the same ways as installing on the command-line.
The ´where´ parameter is optional and only used internally, and it specifies where the packages should be installed to.
The ´packages´ parameter is an array of strings. Each element in the array is the name of a package to be installed.
Finally, ´callback´ is a function that will be called when all packages have been installed or when an error has been encountered.