Add option to display the selected value inside the searchbox

- If the option 'showCurrentValue' is defined as true, when 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'.

- Fix small comment typo.