Support for yard-doctest

Original discussion happened in #264.

I believe I've managed to keep the docs meaningful and not overwhelmed with test details. This however required to skip a bunch of them (see `doctest_helper.rb` for details) and add a lot of hooks for others.

Most of the examples use HTML files from watirspec (except to few).

@jarib @jarmo Please take a look again and let me know what do you think.