feat: add custom operators

[Darkein]

rebase PR #253 with latest master to fix #149

[rgolea]

@zMotivat0r please approve this. It would be a real shame if it gets postponed and delayed like the PR I put out. @Darkein might not be able to keep it synced.

[patrickmichalina]

I was just rolling up my sleeves to write something like this. Can you please comment on if this will be merged?

[Darkein]

I still need this feature, it would be nice if you could have a look @zMotivat0r

[cantoute]

This feature would be gr8 !!! Allowing to implement Postgresql LabelTree for exemple :)

[rgolea]

If only someone could review these changes in time to not have another git conflict 😂

[rewiko]

I hope we can merge a lot of PR soon, if you would like to be a maintainer of this lib, feel free to ask here .
I am a maintainer now, I am happy to review it, but I don't know exactly how to release a new version.
I have discussed with @zMotivat0r, we might want to use https://github.com/zMotivat0r/mrepo to release automatically this lib.

[Darkein]

only 2 small conflicts, it could have been worse :D

[Darkein]

@adrianobnu or @rewiko , could you have a look please ?

[rewiko]

I am trying to help and have forked this repo (can't reach the owner of this repo and do not have the credential for the npm repo).

See #710 (comment)

We can hopefully merge it via rewiko#5