Space as null value flag

I needed slightly different behavior so I thought I would option-ize it. I've set up an additional "options" object parameter on the 3 parse methods (with backwards compatibility for iniparser.parse callback). Also updated readme and added test for the single new option: treatEmptyStringsAsNull, which will, as the name suggests, set the value of parameters that have no direct value (which come in as an empty string) to null.

For example:
```ini
var_with_noValue=
```
Would come in when the flag is false (default) as:
```javascript
{
     var_withNoValue: ''
}
```

and the same thing would come in when the flag is true as:
```javascript
{
     var_withNoValue: null
}
```

It should be relatively easy to add new parameters in the future.