Add option to display the selected value inside the searchbox

In some case the user may want to easely copy & paste the value from select2 to another window or may want to start a new search and use the current value as a starting point.

If the option `showCurrentValue` is defined as `true` and the user clicks on the select2 control, the currently
selected value will be displayed. If there are no selected value then the searchbox remains empty (default behavior). If a placeholder is specified is not treated as the currently selected value. This change only affects the select2 control of type 'single'.

For instance:

```javascript
$("#e1").select2({showCurrentValue: true});
```
A demo is also available <a href="http://jdecuyper.github.io/select2/index.html#DisplayCurrentValueAndAllowItemRepetition" target="_blank">on my blog</a>.