Add waitForSelector support

[andmcgregor]

Hi, is there any interest in merging this change? It just adds support for waitForSelector which I needed on a recent project. Open to any feedback or changes needed.

[abrom]

Seems like a reasonable change. The waitForSelector API does also support some additional options (optional). Thoughts on including support for them too? I can see they might be useful in the same vein (if you wanted to have it wait until say a 'loading' element was hidden instead).

https://github.com/puppeteer/puppeteer/blob/v4.0.0/docs/api.md#pagewaitforselectorselector-options

Maybe something like:

    // If specified, wait for selector
    const waitForSelector = options.waitForSelector; delete options.waitForSelector;
    const waitForSelectorOptions = options.waitForSelectorOptions; delete options.waitForSelectorOptions;
    if (waitForSelector !== undefined) {
      await page.waitForSelector(waitForSelector, waitForSelectorOptions)
    }

The spec for it would likely just have some sort of "loading" element that gets hidden after a period, then expect that the result does not have the loading content.

Can you also please add an entry in the unreleased section of CHANGELOG.md

[abrom]

Although if the options were supported the README would likely also need to be a little more verbose to explain the options

[andmcgregor]

Thanks for the feedback, I added support for passing options in the most recent commit. If more in depth documentation would be preferred in the README let me know- I just linked to the puppeteer documentation for an explanation of the options.

[abrom]

Nice one, thanks @andmcgregor

[abrom]

FYI I've released this in 0.12.2. Thanks for the PR 👍