add git-init prompt

[lukeed]

simple Y/n confirm field

If yes, initializes a git repo, adds all generated/template files, and commits first message:

initial commit, via generator-fly 🚀

marketing

if accepted, closes #10

[lukeed]

@bucaran bump package.json to 0.7.1 or 0.8.0, then publish to npm pls :)

on a different note, can you npm unpublish fly-prefix ? still in the registry

[ghost]

:+1: