bug: new paragraphs i.e. double-newlines

[harry-wood]

Many bbcodes such as [b][s][u] & [i] will allow new paragraphs i.e. double-newlines

[b]para 1

para 2[/b]

and the bold effect will apply to both paragraphs. But this is harder to do in markdown. You can't do this:

**para 1

para 2**

(And this is the current incorrect output)

Ideally it would convert to this

**para 1**

**para 2**

...but don't forget we'd need to re-apply all formatting (any open bbcodes) to both paragraphs, so that may be a bit tricky to implement.

[harry-wood]

If it's any consolation I've just been reporting the same problem in the ruby gem :-)

I'm suggesting a fix there with this commit but it's an ugly cheating workaround, to output this:

**para 1
<br>para 2**