Add easy-to-use binary

Summary: This diff makes ard a binary, so that you can simply say

```
ard pr target
```

rather than

```
thor ard:pr target
```

and also use it in other projects.

Test Plan: dogfoodz

Reviewers:

    @barooo plz
    @gnarmis plz
    @kairuiwang plz
    @kochalex plz
    @lshepard plz
    @mpstreeter plz
    @nhatch plz
    @spo11 plz
    @khsia plz