Add a better interface (help text) for no-longer-supported migrations

[fredemmott]

I'm occasionally deleting migrations if:

• updating them to work with new HHAST is going to be painful
• they've already been in several releases
• they've been required for several releases of HHVM

This generally ends up with CLI flags with documentation of 'no longer supported' and an implementation of throw new ExitException(1, "Use HHAST v4.x.* for this migration");

Some changes to the CLI would be nice for this:

• group together all the obsolete ones in --help - or hide them by default
• instead of requiring the full implementation for each one, perhaps generate the CLIOption flags from a vec<(string $flag, string $first_version, string $last_version)>