Create .eslintrc to resolve https://github.com/reach/router/issues/64

[stunstunstun]

I would like to phase out the issues #64

First of all, We can find that is missing in props validation

.eslintrc

  "rules": {
    "react/prop-types": ["warn", { "ignore": ["children"] }]
  }

If all props are validated, how about replacing the warn level with the error?