Add pre-commit hook to run prettier on staged files

[brunolm]

npm i -D lint-staged husky@^1.0.0-rc.9 prettier

Note: md files doesn't work, remove from the config #19
Note: you need to run git add after prettier f93796a

  "husky": {
    "hooks": {
      "pre-commit": "lint-staged"
    }
  },
  "lint-staged": {
    "*.{ts,tsx}": [
      "prettier --write",
      "git add"
    ]
  },

[coveralls]

Coverage remained the same at 82.278% when pulling 86059a6 on feature/pre-commit-prettier into 7dc2a82 on master.