Post URLs may conflict

It's possible that two posts would generate to the same file.

Given a permalink configuration value of `/:title`, these two files would generate to the same:

```
2000-01-01-my-post
2013-12-31-my-post
```

* Catch it in `serif generate` when generating posts.
* Don't let the admin interface make new drafts that would generate a conflict.

Both of these can just read the permalink value to determine a collision. Note, however, that individual posts can have their own permalinks. (See #46.)