fail in case of adding unique index by default

Hi,
I'm about https://github.com/github/gh-ost/issues/167.
We're faced with the same behavior and it was unexpected for us.
Previously we used `pt-online-schema-change` and it failed and showed a query to check if there were duplicated entries when it detected adding unique index.
Didn't you think about adding flag like `-allow-unique-key` and fail in case of adding an unique key by default?
Or adding something like `-fail-on-adding-unique-key` which will enable checking for adding an unique key.
`pt-online-schema-change`'s `check-unique-key-change` flag documented here: https://www.percona.com/doc/percona-toolkit/LATEST/pt-online-schema-change.html#id7
It would be great to have the same behavior in `gh-ost`.