Markdown filter: Image's title and alt text should be in separate Text Units

[Kuro Kurosaka]

For input file like this:

![alt text should be extracted](http://img.png "title text should be extracted")

tikal.sh -x image-w-alt-and-title.md generates the XLIFF file that consists of just one trans-unit element. The trans-unit/source element looks like:

alt text should be extracted<x id="1"/>title text should be extracted<x id="2"/>

Since the alt text and the title text are shown independently, they should be placed in independent trans-units.

[Kuro Kurosaka]

Lowering priority since translation can still be done.