Allow to append additional salts (e.g. WP_CACHE_KEY_SALT)

[ihorvorotnov]

In addition to #16 it would be nice to allow additional salts, the most important one being WP_CACHE_KEY_SALT, which is used by many object cache drop-ins to guarantee the uniqueness of cache keys.

This wp-cli command does it with additional flag (see the very end of README):

--append=WP_CACHE_KEY_SALT,OTHER_USEFUL_SALT

I'd be happy to bake a PR if you're interested Sorry, that was quite optimistic from my side. Dove into the code and realized it might take more time than I would initially expect :)