Altered filtering behavior

Filtering can be now done on option lables or values or both.
Configurable via: 'filterBehavior' option
Possible values are:
* 'text' (default) - uses only label text for filtering
* 'value' - uses only option value for filtering
* 'both' - uses label text and option value
Merge pull request #96 from nexces/master

Altered filtering behavior.