Add support for KEEPTTL option of SET command

[sam-mosleh]

Fixes #175

[TheKevJames]

Hi @sam-mosleh -- just wanted to let you know that my org has recently forked aredis as it appears to have become unmaintained and we've merged in this pull request (and all other open PRs against the aredis repo); in case you find it useful, the fork is here and version 2.0.0 is on PyPI with your changes.

Also wanted to thank you for implementing this; we've been doing a terrible ttl -> setex thing to try to approximately avoid adjusting some ttls and having this directly in aredis is going to make maintenance so much easier.