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) !== '- ') {` 


Haha I copied it from the other ones - I think we need to change it for a lot of them. Only `leading-underscore-access` says ` it('with object should throws'`.

Maybe reword all to - `should throw with an object` and `should throw with undefined`.