Supporting Markdown for description & changelog

[jlam]

I'd like to request that Markdown support be added to the descriptions and changelogs when uploading a new file for an addon. This is mostly for convenience -- GitHub supports Markdown for the README file in the root of the repository and it's a convenient place to dump the description of a project. If I could just copy and paste its contents instead of maintaining a separate BBCode version of the same text, that would make maintaining my project on WoWInterface a bit easier.