brought forward some MCString methods to ObjC interface

I added the following methods to the NSString (MCO) category.

- (NSString *) mco_flattenHTML;
- (NSString *) mco_flattenHTMLAndShowBlockquote:(BOOL)showBlockquote;
- (NSString *) mco_flattenHTMLAndShowBlockquote:(BOOL)showBlockquote showLink:(BOOL)showLink;

- (NSString *) mco_htmlEncodedString;
- (NSString *) mco_cleanedHTMLString;