Adds dns_opteamax.sh, solving dns-01 challenges through the Opteamax
customer API (api.opteam.ax). Authentication is a Bearer token created in
the customer panel; the zone is detected by walking the name up against the
account's zone list, so subzones and DNS alias mode both work, and rm only
removes the value it was given so a wildcard's two TXT records survive each
other.
