Consider the following case,

zero **one** two

if the keyword is one it will attempt to rebold it, resulting in

zero ****one**** two

which the underlying markdown package does not render as expected.

<p>zero <strong><em>*one</em></strong>* two</p>

Another case,

**zero one two**

would result in,

**zero **one** two**

markdown produces the following html

<p><strong>zero </strong>one<strong> two</strong></p>

which unbolds the keyword instead of bolding it.

This pr attempts to bypass these limitation by excluding already bolded ranges from the replacement.