Add slow tree logging to `broccoli serve`.

Sample output from the Ember Broccolification effort (from `broccoli serve`):

```
% broccoli serve
Serving on http://localhost:4200


Slowest Trees                  | Total
-------------------------------+----------------
DefeatureifyFilter             | 3630ms
DefeatureifyFilter             | 1663ms
TranspilerFilter               | 1610ms
TranspilerFilter               | 647ms

Built - 7955 ms

Slowest Trees                  | Total
-------------------------------+----------------
DefeatureifyFilter             | 92ms
TranspilerFilter               | 88ms
TreeMerger                     | 69ms
TranspilerFilter               | 68ms
Concat                         | 65ms
TreeMerger                     | 57ms
DefeatureifyFilter             | 46ms

Built - 771 ms
```