Allow one line arrow functions

[jhnns]

We currently disallow arrow functions in favor of a consistent function style. They are only allowed within other functions. However, if the function returns immediately (which is very common in functional programming), it is more readable if we use arrow functions:

// Good
const bla = () => true;
// Good
const bla = () => ({
    prop: true
});

// Bad
const bla = () => {
    return true;
};
// Bad
const bla = () => {
    const blub = true;
    return  blub;
};

See also discussion: eslint/eslint#7765

With the https://github.com/TristonJ/eslint-plugin-prefer-arrow plugin, it's finally possible to define that rule:

        "func-style": [
            "error",
            "declaration",
            {
                "allowArrowFunctions": true
            }
        ],
        "prefer-arrow/prefer-arrow-functions": [
            "error",
            {
                "singleReturnOnly": true
            }
        ]

We should do that.

[jhnns]

I prefer a consistent style. Either we use function declarations or arrow functions for everything because refactoring becomes painful if we have both styles. Every time a one-line arrow function becomes a two-line function, I would need to refactor it to a function declaration.

So closing this in favor of #23