Fixed CLI output on serial programatic executions

This commit fixes an issue where calling the `execute` function on the CLI multiple times would produce the incorrect output because the results array was not being cleared out between executions.

Example of the issue:

```js
var cli = require("eslint").cli;

// This execution will display errors.
cli.execute(["file-with-errors.js"]);

// This should pass, but will display the errors from the previous execution.
cli.execute(["passing-file.js"]);
```