Added slightly tweaked support for IE conditionals within build blocks.

Slightly tweaked version of @kavanaghs pull, the regex in kavanaghs seemed to be returning false when checking conditionalBegin. I've rewritten the regex. It works but you have to follow the rules. Any other comments inside or added lines before the opening IE or after the closing IE and it breaks. Indents are ok though. But at least conditionals will work in dev and dist. I think its a good enough workaround for when needed until a more elegant solution is found.

Reviewing the discussion on this the problem with having build blocks within the conditionals is that the scripts will get loaded everywhere when working in dev. The build block cancels out IE conditional to browsers.