Export CLI as a module

So `bin/cleancss` looks something like:

```js
#!/usr/bin/env node

var cleanCssCli = require('clean-css-cli');
return cleanCssCli(process);
```

One could reuse the CLI to do some custom magic:

```js
#!/usr/bin/env node

var cleanCssCli = require('clean-css-cli');
return cleanCssCli(process, function beforeMinify(cleanCssObject) { /* do whatever magic you fancy */ });
```