DROP INVALID INDEXES command

Hi hackers!

Recently Kirill and I have faced several problems with
pg_repack, that resulted in list of invalid indexes on our installations,
and that required us to remove them ourselves.

We thought that it might be useful to have a command in PG, that
removes invalid indexes on specified table automatically, to reduce
the chances of any human-related mistakes. So we implemented a new
command which usage looks like:

postgres=# drop invalid indexes on table sas;
NOTICE: dropping index "idx2"
NOTICE: dropping index "idx5"
DROP INVALID INDEXES
postgres=#

We would like to know if this feature would be acceptable
for upstream inclusion.
POC patch attached, any feedback on the design and implementation is welcome :)

--
Best regards,
Roman Khapov