Improve .gitignore util by appending to .gitignore if file already exists

Great little util for using [gitignore.io](http://www.gitignore.io).

Just improved it a little by appending new types to existing .gitignore file.
I also remove the duplicate `# Created by http://www.gitignore.io` line when appending.

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[Gist](https://gist.github.com/matiassingers/8248b4345fb56b8a08da)