Parser fails to identify HRule

[jyutzler]

Send the following into the parser

Hello World
***

---
___

These should all be an HRule:

---

---

---

None of them work right in pegdown-1.4.2.jar

[vsch]

You have to have at least one blank line above and below an HRule marker. GitHub's rules are more relaxed.

try:

Hello World

***

---

___

It is actually a bit less strict on the blank line above. You need it if the previous lines can include lazy continuation that could interpret the horizontal rule as that. So to be safe it is best to have a blank line before the HRule.

[sirthias]

Closed by #181.