Topological order

The idea is to sort addins in a topological order. Each addin can define one of five things for _order_:
* `>X` - I must be immediately after the addin with id X

* `>>X` - I must be somewhere after the addin with id X (but not necessarily immediately) 

* `<X`- I must be immediately before the addin with id X

* `<<X` - I must be somewhere before the addin with id X (but not necessarily immediately) 

* `Number` - A numerical order which is used if none of the above is applicable


var firstIndex = _.findIndex(this.addins, {id: firstId});
var secondIndex = _.findIndex(this.addins, {id: secondId});
_.each([firstIndex, secondIndex], function (index) { if (index == -1) { throw new Error (...) })