Allow perftest to run an arbitrary directory

So I can point them at the compliance tests or any arbitrary directory:

```
$ python perf/perftest.py -d tests/compliance/
parse_time: 5.76305389ms, search_time: 0.00095367ms description: tests/compliance/basic.json-0 name: foo
parse_time: 5.78117371ms, search_time: 0.00190735ms description: tests/compliance/basic.json-0 name: foo.bar
parse_time: 5.87606430ms, search_time: 0.00286102ms description: tests/compliance/basic.json-0 name: foo.bar.baz
parse_time: 5.88297844ms, search_time: 0.00476837ms description: tests/compliance/basic.json-0 name: foo.bar.baz.bad
parse_time: 5.85913658ms, search_time: 0.00286102ms description: tests/compliance/basic.json-0 name: foo.bar.bad
```