Tell It's All Text! that file textarea is markdown.

The [It's All Text!](https://github.com/docwhat/itsalltext) Firefox extension
lets you edit textareas in your favorite desktop editor.

This change tells IAT that it should use an extension of `.md` when creating
the file for the desktop editor; which allows syntax highlighting and
indenting to work in most editors.

FYI: I'm the author of IAT.  I'm using this plugin for Q2A and
like it very much.  This change makes it even more useful for
me and my users. :-)