Summary:
**Summary**

This PR, similarly to https://github.com/facebook/react-native-website/pull/1337, adds [alexjs](https://alexjs.com/) linting to Draft.js in order to identify and prevent insensitive and inconsiderate language from landing on master.

- Adds Alex
- Configures Alex rules
- **TODO:** Run Alex in Travis CI

**Test Plan**

- Run `yarn lint-docs`
- Verify there are no lint violations
Pull Request resolved: https://github.com/facebook/draft-js/pull/2223

Differential Revision: D18046996

Pulled By: claudiopro

fbshipit-source-id: 66ac935720a0b13576e9a133a3d5f8e98cc539cf