new rule: requireHyphenBeforeDescription

#50

This might need some more tests?

I'm assuming most people want the space so its like ` - description` not ` -description`

so it works like ` if (tag.description.substring(0, 2) !== '- ') {` 


Doesn't look jscs has any way of testing the position of the error pointer right now? Since all of our tests in jscs and here are checking if error = 0 or 1.