module-prefix defaults to npm install prefix

The npm install prefix seems to be the most dependable and robust default for module-prefix.

With this PR we will look for modules in ```{prefix}/node_modules``` and ```{prefix}/lib/node_modules```. I used https://www.npmjs.org/doc/files/npm-folders.html#node-modules as a reference.

This I believe would catch these OS defaults:
 * Mac: ```/usr/local/lib/node_modules```
 * Ubuntu: ```/usr/lib/node_modules```
 * Windows: ```{prefix}/node_modules``` (whatever that prefix may be)

It would also allow ndm to support custom npm install prefixes either set by the user or another platform.