Added support for strikethrough text and fixed bugs

Fixed bugs on bold, italic and strikethrough tags according to the following rules:
* Whitespace after opening and before closing an tag is invalid ( e.g. _ text_ or _text _ ).
* Before opening and after closing tags the only character accepted is anything but a word or a digit ( 2*text* and d*text* are invalid but $*text*$ is).
* Adding the same character after a opening tag is invalid but not before it closes (e.g. ~~~invalid~~ and ~~valid~~~).