Add support for expire-related command options nx, xx, lt and gt

[viralpraxis]

Resolves #286

I also addes some specs to run against redis 7. Some of the existing specs fail, so I marked new specs with redis: 7.0 tag.

Running bundle exec rspec --tag redis:7.0 should run only redis-7-related specs, and running with redis:6.2 or without tag at all should run all the specs except of redis-7-related. Not sure if this is an appropriate/optimal solution

[sds]

This is great, thank you! Sorry for the delay in merging.