Only remove options if a loading placeholder is available

Currently, when `isLoading` is set to true, the options array
is set to an empty array and we show the loading placeholder instead.

Our app has an async service which responds in a couple of ms,
resulting in the autocomplete list flickering in and out of existence
as you type.

This change allows users to opt out of the 'clear options while loading'
behavior by not providing a loading placeholder.

src/Async.js

@@ -8,7 +8,7 @@ const propTypes = {
 	children: React.PropTypes.func.isRequired,       // Child function responsible for creating the inner Select component; (props: Object): PropTypes.element
 	ignoreAccents: React.PropTypes.bool,             // strip diacritics when filtering; defaults to true
 	ignoreCase: React.PropTypes.bool,                // perform case-insensitive filtering; defaults to true
-	loadingPlaceholder: React.PropTypes.oneOfType([  // replaces the placeholder while options are loading 
+	loadingPlaceholder: React.PropTypes.oneOfType([  // replaces the placeholder while options are loading
 		React.PropTypes.string,
 		React.PropTypes.node
 	]),
@@ -146,7 +146,7 @@ export default class Async extends Component {
 		const props = {
 			noResultsText: isLoading ? loadingPlaceholder : searchPromptText,
 			placeholder: isLoading ? loadingPlaceholder : placeholder,
-			options: isLoading ? [] : options,
+			options: (isLoading && loadingPlaceholder) ? [] : options,
 			ref: (ref) => (this.select = ref)
 		};