Create the .gitignore file as part of the installation

When `Distill::extractWithoutRootDirectory` is called from the `new` command, it uses the `SKIP_DOTS` constant in the `FilesystemIterator` which causes the `.gitignore` file not to be included as part of the installation. This is not configurable in Distill at this time.

I'm not sure whether this is the best method to accomplish this, since it potentially requires maintenance of the `$ignored` array, I'll gladly take suggestions...