Configure Puppeteer's page-level interaction timeout

Hi @garris, I'm wondering if we could introduce an option to configure the default timeout for page interactions, that would normally be set directly in Puppeteer `page` methods as a timeout option.

Since we are allowing `waitTimeout` to be passed in the `engineOptions` property (despite being a non-valid property that `puppeteer` would accept directly) and having it used here: https://github.com/garris/BackstopJS/blob/b45fab38661056ad94abff26b2ee87ff0157a8d0/core/util/runPuppet.js#L67

Can we add an option and potentially pass that value to `page.setDefaultTimeout()`, and place it after the line pasted above? Then we can set a timeout higher than 30 seconds for any page-level interactions like waiting for a selector?

Thank you for your work on this, and I look forward to hearing your feedback!