`.gitignore` not being copied by extension creator

The `/.gitignore` file from the extension boilerplate doesn't get copied to the created extension, leading `node_modules` to be added to git.

This is because npm removes `.gitignore` files when publishing packages, meaning that even when `globOptions.dot` is `true` when calling `copyTpl`, the file won't be copied.

The [recommended solution](https://github.com/yeoman/generator/issues/812) appears to be to rename the files to something else, such as `gitignore` and renaming them to `.gitignore` after copying them.