Allow different symbols for em and strong

[justanotheranonymoususer]

strong_em_symbol allows to define a symbol for both, but not separately.

Here's a problem: consider:

<em>a</em><strong>.b</strong>

Here's how it can be converted to markdown and the commonmark interpretation:

<!--
<em>a</em>**.b**
-->
*a***.b**

<!--
<em>a</em>__.b__
-->
_a___.b__

<!--
<em>a</em><strong>.b</strong>
-->
_a_**.b**

https://babelmark.github.io/?text=*a***.b**%0A%0A_a___.b__%0A%0A_a_**.b**

As you can see, only the third option works, but the lib doesn't provide such option.