Cleanup save_* family of methods

Follow up to #1273 and #958

* Better YARD documentation
* Fix bug in `inject_asset_host` when `asset_host` is set and `base` tag is present
* Make `path` argument of `save_screenshpt` `nil` by default
* Puts generated filename to `$stdout` when `save_page` or `save_screenshot`
are invoked without args