Mixed emphasis markers

[michelf]

Input:

 **This** *works*

 **This_doesn't** *work*

Output:

(https://michelf.ca/projects/php-markdown/dingus/)

<p><strong>This</strong> <em>works</em></p>

<p><strong>This_doesn't</strong> *work*</p>

Expected:

(http://daringfireball.net/projects/markdown/dingus)

<p><strong>This</strong> <em>works</em></p>

<p><strong>This_doesn't</strong> <em>work</em></p>