[#75177816] Accepts a username and password for HTTP Basic Auth

Some sites that we crawl might require a username and password (via
HTTP Basic Auth) for authentication. This change allows setting these
values through environment variables and passes them through to the
crawler for use in the HTTP transport.
Alphabetise the list of ENV config variables
Merge pull request #24 from alphagov/accepts-username-and-password-for-http-auth

[#75177816] Accepts a username and password for HTTP Basic Auth