Support importing from npm packages

This adds support for importing from npm packages installed to node_modules.  For example, having installed `twbs` via npm you can import the entirety of bootstrap using:

```less
@import (npm) "twbs";
```

or a specific file via:

```less
@import (npm) "twbs/less/code";
```

The syntax and resolution algorithm will be familiar to most node.js developers, and is becoming increasingly popular on the client side too with libraries such as browserify.

closes #1965