Add .wait until <event> fired

[0xgeert]

Consider the use case where we wait until a certain event is fired. For example a really useful one is the following:

const html = await chromeless
    .goto('https://www.github.com')
    .wait('DOMContentLoaded')
    .html()

Note: Lifted this from #74 (comment) by @Kikobeats

[joelgriffith]

Once something like #110 lands we'll have a clear path for this sort of thing. From that we can (or consumers) can write higher-level methods using it, like DomContentLoaded

[nasa8x]

I use .wait('DOMContentLoaded') but not working. How do I write code right?