Adjust lint rules

[achichen]

1. Allow confusing arrow function with parenthesis
   this syntax would violate no-confusing-arrow rule before this version:

// eslint will reject
const toDoOrNotToDo = (condition, todo) => condition ? todo : null

now it can be avoided by surrounding parenthesis:

// this is good
const toDoOrNotToDo = (condition, todo) => (condition ? todo : null)

1. No newline is required before return
2. Allow mixed operator, e.g. 3 + 4 * 5