cli: Provide an init command

As seen in #47 setup currently easily allows for errors in documentation and frustration for users. Add an `commitlint init [--extends]` command. It should

* [ ] create a new `commitlint.config.js` file if needed, prefer `package.json` if available
* [ ] ensure `@commitlint/cli`
* install/update `@commitlint/cli` as needed
* [ ] add configs provided via `--extends` to `module.exports.extends`
* default to `@commitlint/config-conventional`
* support multiple `--extends`
* check if values are available if npm ids
* install/update `@commitlint/config-*` as needed
* disable via `--no-config`
* [ ] ensure `husky`
* ensure presence of commitmsg run-script
* disable via `--no-husky`
* [ ] add `@commitlint/travis-cli` to `.travis.yml` if file is found and not in scripts
* install/update `@commitlint/travis-cli` as needed
* disable via `--no-travis`

* ~~optional: respect .editorconfig?~~ overkill

Start by reading issue #47 and the existing CLI entry points, then trace how package.json, commitlint.config.js, and .travis.yml are handled. The work is complete when commitlint init supports the listed config, dependency, Husky, Travis, and disabling options, with tests covering the requested combinations.

Written by the indexing model from the issue text.