Implement mouseover

[Nedomas]

Really needed mouseover feature so just copied @ronanyeah code from this PR https://github.com/graphcool/chromeless/pull/238 onto current master. It still works, no conflicts. Unclear why that is not merged yet, but there was no progress for the last year, so reopening it here.

Plan is to check if tests pass, write some new ones that cover this and possibly have it on the next release.

• [ ] If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• [ ] Make sure all of the significant new logic is covered by tests
• [ ] Rebase your changes on master so that they can be merged easily
• [ ] Make sure all tests and linter rules pass
• [ ] If you've changed APIs, update the documentation in README and /api/README

[ronanyeah]

@Nedomas I closed it because https://github.com/GoogleChrome/puppeteer got released and I started using that instead, but glad you found the code useful. :+1: