Feature request: allow auto-add of Gist-ID comment for recognized filetypes

[Kazark]

Matt, I love your plugin. Whenever I use Gists, it makes my life better.

Almost every time I create a gist in Vim (every time?) I like to add the gist ID comment in the header so that it will automatically be correlated to the correct Gist on save. This is an extremely nice feature. However, I have to do it manually and I'm always nervous I haven't remembered the syntax correctly.

It might be nice to allow an opt-in behavior of adding that ID comment after initially posting for recognized filetypes (in my case, particularly Markdown and VimScript).