Markdown filter: add an option to toggle extraction of image alt text

Markdown images can support both alt text and a title:

![alt text](https://github.com/adam-p/markdown-here/raw/master/src/common/images/icon48.png "Logo Title Text 1")

Currently we extract both alt text and title. Not everyone localize alt text, so we should make this an option. The text will be extracted by default.