Updated initWithHTMLString can set customURL

- (id)initWithHTMLString:(NSString*)HTMLString;
When loading web only from HTMLString and didn’t set customURL, the
actionButton will be disable.

- (id)initWithHTMLString:(NSString*)HTMLString customURL:(NSURL*)URL;
When loading web from HTMLString and have set customURL, the
actionButton will be enable, and use customURL to share URL.