Only trim() if an option is set.

I found it surprising that `chomp` trimmed leading and trailing whitespace from each returned line, so I added an option to control it. It defaults to `true` for backward compatibility.

```javascript
var chomp = require('chomp').chomp;

chomp('somefile.txt', { trim: false }, function (err, lines) {
  console.log(lines);   // => lines include leading and trailing whitespace
});
```