Add tachikoma init command

This command provides quick bootstrap!

Usage:
$ bundle exec tachikoma init

Create `Rakefile`, `data/`, `data/__example__.yaml`. `repos/`, `repos/.gitkeep`, `.gitignore`.
```text
.
├── Gemfile
├── Gemfile.lock
├── Rakefile
├── .gitignore
├── data
│   └── __example__.yaml
└── repos
　   └── .gitkeep
```