Markdown filter: inline markups inside the link text are not replaced by place holders

[Kuro Kurosaka]

The anchor text in the inline link can have markups such as:

This is a [link with *emphasis*](http://google.com) that should be treated just like a *emphasis* outside of the link text.

Whether *emphasis* is outside or inside the square brackets, these asterisks need to be replaced by place holders. But the current implementation treat *emphasis* as a plain text and doesn't escape the asterisks. The generated trans-unit/source looks like this:

This is a <x id="1"/>link with *emphasis*<x id="2"/> that should be treated just like a <x id="3"/>emphasis<x id="4"/> outside of the link text.

where it should look like:

This is a <x id="1"/>link with <x id="2"/>emphasis<x id="3"/> <x id="4"/> that should be treated just like a <x id="5"/>emphasis<x id="6"/> outside of the link text.

The same is true for the reference link, but due to issue #711, the text is not extracted at all.

[Kuro Kurosaka]

The fix has been merged in pull request #234.