Add y/n prompt to sub command init

[masutaka]

Related to #59

#58 (comment) で話した件。

$ github-nippou init では有無を言わさず

1. Gist を作成
2. ~/.gitconfig への書き込み

が行われるため、確認のプロンプトを出すようにした。

y

$ bundle exec bin/github-nippou init
This command will create a gist and update your `~/.gitconfig`.
Are you sure? [y/n] y
The github-nippou settings was created on https://gist.github.com/1234abcd

And the gist_id was appended to your `~/.gitconfig`. You can
check the gist_id with following command.

    $ git config --global github-nippou.settings-gist-id

n

$ bundle exec bin/github-nippou init
This command will create a gist and update your `~/.gitconfig`.
Are you sure? [y/n] n
Canceled.