add ssl_opts

applied @xaicron's patch and documented it. solving #39.

Note that implicit SSL_verify_mode is deprecated and IO::Socket::SSL says, "In the near future the default will be SSL_VERIFY_PEER" (section v1.79 of Changes). Thus, We'd better set SSL_verify_mode to SSL_VERIFY_PEER in Furl::HTTP in the near future, too.