allow onChange to trigger when calling `select` or `deselect` with one value

See https://github.com/davidstutz/bootstrap-multiselect/issues/312; when you call `select` or `deselect` with only one option value, you are effectively mimicking the user clicking on one of the checkboxes, so it makes sense to trigger `onChange` for that particular option.

This allows doing:
```javascript
.multiselect('select', optionValue) // won't trigger onChange
.multiselect('select', optionValue, true) // will trigger onChange
```

Not sure how I feel about passing an extra optional argument to every multiselect method, but it seems harmless, ensures backwards compatibility, and may provide more flexibility in the future.