Hello,

I am using Mongo DB as storage, but not rest-layer-mongo.

I want to implement Contains and Does Not Contain filters. For Contains I can use the $regex operation. However, for Does Not Contain I cannot use $regex, since Go does not support lookarounds and regexes are currently validated with Go's standard library. Using $not seems to me like the obvious alternative.

Per #272 I know that there is not active development on this library, so this issue is to ask if a reason exist for this feature to be missing. If there is no such reason, I will implement it myself (including support for $not-ing a regex and $not-ing a whole expression).
#237 looks like an appropriate PR to model after