Add require() helper

### Node.js `require()` helper

This code maps the commands `:Require` and `:Unrequire` to help with managing `require()` statements in Node.js code.  Commands `:R` and `:UR` are also created if they don't already exist.

### A brief usage example, starting in a blank node.js file:

Run these vim commands:

    :Require async
    :Require cp=child_process
    :Require ./lib/somelib
    :Require mongodb.MongoClient
    :Unrequire mongodb
    :Require mc=mongodb.MongoClient

The file will go through these states:

```
    var async = require('async');
```
```
    var async = require('async'),
        cp    = require('child_process');
```
```
    var async   = require('async'),
        cp      = require('child_process'),
        somelib = require('./lib/somelib');
```
```
    var async       = require('async'),
        cp          = require('child_process'),
        MongoClient = require('mongodb'),
        somelib     = require('./lib/somelib');
```
```
    var async   = require('async'),
        cp      = require('child_process'),
        somelib = require('./lib/somelib');
```
```
    var async   = require('async'),
        cp      = require('child_process'),
        mc      = require('mongodb').MongoClient,
        somelib = require('./lib/somelib');
```

### Improvements

This has been very useful to me so far, but it's my first piece of serious vimscript, so changes and improvements welcome!