Emphasis and "the rule of three"

[mity]

The following input:

***foo *bar baz***

is currently translated to

<p>***foo <em>bar baz</em>**</p>

However it should really be

<p>*<strong>foo <em>bar baz</em></strong></p>

Rationale: The rule of three does not apply when both the opening and closing delimiter run have the lengths which are multiplies of three.

[mity]

This seems to be related to #98: Its fix seems to be incomplete.