Triple-asterisk leads to unmatched STRONG tag

```
>> markdown "***FOO***"
== "<strong>*FOO/**"
```

(What it's supposed to do is be both bold and italic.)

Generally it seems there should be some sort of protection or assert against making unpaired tags in a single call to markdown, as that allows errors to propagate pretty far in the document (perhaps some way of designing it so that can't happen?)
