Fix <hr> insertion on double <p>

Regarding #27 and #31, it was almost on parity with Medium; I just found a small bug, when pressing enter at the start of a non empty paragraph, preceeded by another non empty paragraph; on Medium this inserts an `hr`, on grande it inserted a regular `p`.

So here's a patch addressing that, but it's probably a good idea to check that it covers all situations.