Adds a friendly goodbye instead of erroring

There’s probably a better spot to put this, but I thought Surge should exit quietly if you changed your mind about a deployment instead of giving an error:

```sh
TypeError: Cannot read property 'project' of undefined
    at /Users/kennethormandy/Sites/kennethormandy/surge/lib/middleware/project.js:24:15
    at /Users/kennethormandy/Sites/kennethormandy/surge/node_modules/prompt/lib/prompt.js:316:20
    at /Users/kennethormandy/Sites/kennethormandy/surge/node_modules/prompt/node_modules/utile/node_modules/async/lib/async.js:136:21
    at assembler (/Users/kennethormandy/Sites/kennethormandy/surge/node_modules/prompt/lib/prompt.js:282:18)
    at /Users/kennethormandy/Sites/kennethormandy/surge/node_modules/prompt/lib/prompt.js:322:20
    at /Users/kennethormandy/Sites/kennethormandy/surge/node_modules/prompt/lib/prompt.js:510:14
    at onError (/Users/kennethormandy/Sites/kennethormandy/surge/node_modules/prompt/node_modules/read/lib/read.js:94:12)
    at Interface.<anonymous> (/Users/kennethormandy/Sites/kennethormandy/surge/node_modules/prompt/node_modules/read/lib/read.js:66:5)
    at Interface.EventEmitter.emit (events.js:92:17)
    at Interface._ttyWrite
```

Then, I realised we could say a friendly goodbye instead. This is just an example for now, but later we could do some cool stuff with it, ex. random messages, last Tweet…I dunno.

To try it out:

```sh
./lib/cli.js
# Now press ctrl+c
```