Fixes #4.

This PR adds the feature eslint --init into eslint-cli.

People can set up ESLint for their project with eslint --init command. The command installs eslint and necessary plugins into the project local.

However, eslint-cli requires local-installed ESLint to use eslint --init. This is inconvenient.

# Do `eslint --init` in order to install ESLint locally!
$ eslint --init
Cannot find local ESLint!
Please install ESLint by `npm install eslint --save-dev`.

This PR solves the problem.
Users can use eslint --init in order to install ESLint locally.

Also, this should help us that we remove --init feature from ESLint core to reduce core size in future.

I did refactoring this feature because it has some complex and huge functions.
This is file structure: https://github.com/mysticatea/eslint-cli/tree/init/lib/init