markdown: Use named parameters to add_a helper.
markdown: Use named parameters to add_a helper.

This has enough parameters that it benefits from making which is which
explicit.

zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip