Markdown support

I have added support for the Markdown markup language to OS4Depot readmes.

You can select if your readme text should be displayed as plain text as before or as Markdown on the submit file page.

Comments now default to Markdown as well.

There is a new feature in the left menu Markdown composer where you can test how your readme will be displayed in Markdown mode before submitting your file.

Please let us know if there is a problem with submitting files or with the Markdown formatting.

Do note that images and tables are not supported in the markdown mode.