Feature Request: Add all files with specific extension to .gitignore.

[fpnick]

In the Magit buffer, when the cursor is on an untracked file, the 'I' key will add that file to .gitignore, see #146 .
In some situations it could be useful to add all files with the same extension to .gitignore. I.e. if the cursor is on the file foo.bar, add the line *.bar to .gitignore.

[jreybert]

What could be the mapping for this new feature?

I would like to make it kind of composable: ignore file would become II, and ignore extension would be IE.

I will still be mapped to ignore file, but there will be a delay after the keystroke as there will mappings I + something.