Add a way to annotate a callback

[gajus]

Example:

import cheerio from 'cheerio';

type CallbackType = (element: cheerio, index: ?number): any => {};

export default (selector: cheerio, callback: CallbackType): Array<string> => {
    const values = [];

    selector.each((index, element) => {
        values.push(callback(element, index));
    });

    return values;
};

[phpnode]

The annotation should work as in it verifies it is a function but it seems impossible to verify args and return type in a non intrusive way. Maybe wrapping in another fn would be OK but there's definite edge cases where it would do the wrong thing.

[phpnode]

works in flow-runtime