Exec action pipeline

[eoussama]

Users should be able to resolve an exec action with an action array that should execute before any chained actions.

Example:

// e is a reference to the tw instance
tw.exec((e) => new Promise(resolve => {
    const hash = Math.floor(Math.random() * Math.pow(0xffffff, 2)).toString(36);
    resolve([e.type.bind(hash), e.delete.bind(hash.length)]);
})).type("Lorem Ipsum");

Subtask of #67