Setup guards for config options

[tcbyrd]

Closes #81

I wasn't sure what the most optimal way to do this was, but here's what I came up with. If we check the config in the constructor and throw the appropriate errors, it won't attempt any further action on the offending repository until the next scheduled markAndSweep. We still need a way to report this error to the user, but this at least handles the error more gracefully and lets us know the problem was caused by an invalid configuration.

Here is an example error message:

00:12:05.784Z ERROR probot: daysUntilClose must be between 1 and 10000: (daysUntilClose=10005)
  RangeError: daysUntilClose must be between 1 and 10000: (daysUntilClose=10005)
      at checkConfig (/Users/tcbyrd/probot/stale/lib/stale.js:10:13)
      at new Stale (/Users/tcbyrd/probot/stale/lib/stale.js:30:5)
      at forRepository (/Users/tcbyrd/probot/stale/index.js:57:12)
      at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:160:7)
  --
  event: {
    "event": "schedule.repository",
    "repository": "tcbyrd/testrepo",
    "installation": 32078
  }