Respect base tags

[noyainrain]

If a document defines a base tag, take it into account when resolving extracted ad URLs.

Draft

class Company:
    """
    URL field of an ad. The extracted URL is resolved against the base URL of the document (for HTML
    see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base).
    """
    url_path: str

Implementation hints: Company._parse_html() could search for a <base> tag and join it with each
ad URL. A minimal HTML document containing a <base> tag could be used for a unit test.

[noyainrain]

I'm going to implement this live on https://www.twitch.tv/noyainrain next Thursday at 18:00 CET (12 pm EST) 😊

@l3d00m