Full Node.js require() IntelliSense

IntelliSense in `require()` now shows completions for folders and files within the current path segment.
The `/` key behaves exactly as you would expect, re-triggering completion for the next directory.

This also contains general improvements to string completion (see commit details).

![module subdir completion](https://f.cloud.github.com/assets/313894/1270201/24751cfa-2d06-11e3-8fd5-94a7e44d56ae.png)
![local file completion](https://f.cloud.github.com/assets/313894/1270202/247b14f2-2d06-11e3-9dff-ff78ae9803e3.png)