Add support for strikethrough text

Uses a "strike" class
Fixed several bugs on bold, italic and strike tags

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 not $*text*$ ).
Oops, removed the strike tag

Moved it up too
Fixed a bug on bold, italic and strike tags

Adding the same character in after a opening tag is invalid but not before it closes (e.g. ~~~invalid~~ and ~~valid~~~)
Fixed and improved regex
Added specs for strikethough a minor thingy
Merge pull request #38 from asantos3/master

Added support for strikethrough text and fixed bugs