Exit gracefully when SLACK_TOKEN is not set

[osxi]

I read the README but decided I'd try node main.js anyway and got this error:

TypeError: Cannot read property 'map' of undefined
    at /home/zach/code/terminal-slack/main.js:77:17
    at Request._callback (/home/zach/code/terminal-slack/slackClient.js:34:26)
    at Request.self.callback (/home/zach/code/terminal-slack/node_modules/request/request.js:368:22)
    at emitTwo (events.js:87:13)
    at Request.emit (events.js:172:7)
    at Request.<anonymous> (/home/zach/code/terminal-slack/node_modules/request/request.js:1219:14)
    at emitOne (events.js:82:20)
    at Request.emit (events.js:169:7)
    at IncomingMessage.<anonymous> (/home/zach/code/terminal-slack/node_modules/request/request.js:1167:12)
    at emitNone (events.js:72:20)

It would be better if the user had a friendly message and followed by process.exit(1).

[evanyeung]

This was fixed in commit c21b181. Thanks for bringing it up!