Warn on slow initializers.

**Not ready for merge.** There are a couple of outstanding questions in comments. Advice welcome! Just wanting to see if there is interest first.

This is a common case of slow app boot time but is hard to discover otherwise. Using Rails.logger here is insufficient, since it doesn't yet get sent to the console and will be hidden in the log where it won't be seen.

Output looks like:

```
[WARNING] Initializer sass.rb took 0.20s to complete (> 0.10s)
```