Configure additional headers (cache-control, accept, accept-language, etc.)

[dblock]

Some sites 404 or 403 on bots. Add more headers to pretend that we're not a bit and make the headers configurable.

A good set seems to exist in https://gitlab.com/ZakCodes/jekyll-link-checker/-/blob/master/lib/link-checker.rb#L11

 HEADERS = {
    "User-Agent" => "Mozilla/5.0 (Windows NT 6.1) " \
                    "AppleWebKit/537.36 (KHTML, like Gecko) " \
                    "Chrome/41.0.2228.0 Safari/537.36",
    "Accept" => "text/html," \
                "application/xhtml+xml," \
                "application/xml;" \
                "q=0.9,*/*;q=0.8",
    "Accept-Language" => "en-US,en;q=0.5",
    "DNT" => "1",
    "Upgrade-Insecure-Requests" => "1",
    "Pragma" => "no-cache",
    "Cache-Control" => "no-cache"
  }.freeze