Module `crawlmi.utils.lxml_fix`, method `get_html()`:
    - handle `str` objects
    - restrict responses to `HtmlResponse`
    - handle empty responses
Added `extract_links_from_html()` method to LxmlLinkExtractor.
Added `nofollow` field to Link object.
Added `spargs` option to `crawl` command.
Created specialized HTTPS download handler. It tries multiple SSL methods
for the connection before giving up.