Follow <embed> tags as <a> tags

[thequbit]

Noticed that it may be useful to follow tags the way tags are followed. For tags we follow the href attribute, with tags we follow the src attribute.

This should be included as a possible boolean value passed into the barking owl scraper when initialized.

added support for:

tag_types = [
            ('a','href'),
            ('img','src'),
            ('link','href'),
            ('object','data'),
            ('source','src'),
            ('script','src'),
            ('embed','src'),
            ('iframe','src'),
        ]

Added in 0.5.2