set anonymous at voting start

Seems a bit fast and loose to allow the votemaster to choose _after_ voting has commenced whether the results are anonymous or not?

I would think it'd be a param to `startvote`, i.e.

```
you: /startvote anon
slackbot: azharb has started an anonymous vote. vote now!
you: /reveal
slackbot: here are the votes: ...
```

PS: I haven't installed this (yet), just creeping on GitHub. :octocat: 
