add conditional enable to bootstrap-select

Extended bootstrap-select to support dependencies - i.e. enable/disable a <select>, <optgroup>, <option> based on the evaluation result of a jQuery selector string.

## Usage

Use "enable-if" attribute on the desired <select> or <optgroup> or <option>. The value of "enable-if" should be a jQuery selector expression. You can also use "&&" as logic AND. Nesting is not supported. If no "enable-if" attribute is provided or if its value is empty, by default the element will be *enabled*.

## Examples

```html
<select id="platform">
	<option>--- Select Platform ---</option>
	<option value="android">Android</option>
	<option value="ios">iOS</option>
</select>

<input type="checkbox" id="all-platforms"/> <label for="all-platforms">All Platforms</label>

<select>
	<option>--- Select Model ---</option>
	<optgroup label="Android" enable-if="#platform option[value='android']:selected, #all-platforms:checked">
		<option>Galaxy Note III</option>
		<option>Galaxy S4</option>
		<option>HTC One</option>
	</optgroup>
	<option enable-if="option[value='ios']:selected">iPhone 5S</option>
	<option enable-if="#platform option[value='ios']:selected && #all-platforms:checked">iPhone 5</option>
</select>
```