* Check to make sure default npm exists at path before trying to use it
**Bug**
If the typings installer is run under a copy of node that does not include npm—but on a machine that does have npm installed—it will incorrectly try to use the npm that does not exist next to its running version of node
**Fix**
Make sure that we check that npm we select exists before trying to use it as the default. Otherwise, fall back to using plain old `npm`
* Add command line flag to gate new behavior
* Fix missing semicolon
