Make it possible to specify a test file to run

With this patch, we now are able to run a test like:

```
$ bundle exec ruby test/plugin/test_in_status.rb
```

TIPS:

We can specify a line number like:

```
$ bundle exec ruby test/plugin/test_in_status.rb --location 24
```

We can find help like:

```
$ bundle exec ruby test/plugin/test_in_status.rb -h
```