Your comments

Ah, sorry, I thought you were talking about the iOS version. The Mac version doesn't have Markdown preview and export support at the moment.

You can use the excellent Marked app for this purpose: http://marked2app.com
You can already do that: when you have a Markdown file open, you can select the action button and "Copy generated HTML". This will copy the HTML code to the clipboard.
I plan to release it together with the iOS 7 update. I don't have a specific date for you at the moment.
I plan to add support for the new TextExpander SDK in the next update.

Actually, this board is very active. Have a look at the "Latest Updates" section at http://feedback.textasticapp.com or at http://feedback.textasticapp.com/list/1158-textas...
The code completions files can be found at https://github.com/blach/Textastic-Customization

You can modify the "br" behavior by changing the "defaultAppend" string in line 165 (https://github.com/blach/Textastic-Customization/b...

Syntax definitions can be found by searching for "<languagename> tmbundle". Textastic supports textmate 1 syntax definitions. For example, the HTML syntax definition can be found at https://github.com/textmate/html.tmbundle/tree/textmate-1.x
I think that's outside the scope of Textastic. It would propably be more appropriate for a separate app.
I'm using the following TextMate bundle in Textastic: https://github.com/textmate/fortran.tmbundle

It doesn't seem to support these obsolete features and I don't plan to add it, sorry.

You can add custom syntax definitions to Textastic by using TextMate bundles. See
You could try to modify the Fortran bundle yourself. The syntax files are located here: https://github.com/textmate/fortran.tmbundle/tree/...

The Fortran (Modern) comment syntax is defined here: https://github.com/textmate/fortran.tmbundle/blob/...

It supports "!" and "!-". The difference is that comments with "!-" also appear in the symbol list.