add .exit to list of REPL commands

Although Control-D can be used, for some users it would
be good for them to know that they can also just type
`.exit`
fix readme, REPL config in wrong section
add replHelp and replContext, fix .exit

add config options for REPL

* `replHelp` - Array of additional text lines to add to repl `help` command

```javascript
var config = {
  replHelp: [
      'process     - access node.js process',
      '.break      - interrupt current command'
  ]
};
```

* `replContext` - Object of additional properties or functions to add to the REPL context

```javascript
var config = {
  replContext: {
    foo: fooObject  // adds foo to the REPL which exposes the fooObject
  }
};
```

fix .exit since it was not closing properly