Add --init option to rubocop command

[koic]

This PR adds --init option to rubocop command.

% mkdir example
% cd /tmp/example
% rubocop --init
Writing new .rubocop.yml to /private/tmp/example/.rubocop.yml
% cat .rubocop.yml
# The behavior of RuboCop can be controlled via the .rubocop.yml
# configuration file. It makes it possible to enable/disable
# certain cops (checks) and to alter their behavior if they accept
# any parameters. The file can be placed either in your home
# directory or in some project directory.
#
# RuboCop will start looking for the configuration file in the directory
# where the inspected file is and continue its way up to the root directory.
#
# See https://github.com/rubocop-hq/rubocop/blob/master/manual/configuration.md
% rubocop --init # A warning will occur when .rubocop.yml already exists
.rubocop.yml already exists at /private/tmp/example/.rubocop.yml

It may be good to add a simple commented examples of basic configuration in the future.

First of all, this PR aims to generate .rubocop.yml which contains the following configuration URL.
https://github.com/rubocop-hq/rubocop/blob/master/manual/configuration.md

And the generated .rubocop.yml text is quoted from the above URL.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Good idea!

[koic]

This idea was proposed by rubocop/rubocop-jp#34 (Japanese text). Thanks, @sinsoku 👍