Make condition operators extendable

[myguidingstar]

Currently operators must be one of =, >, < etc. Users should be able to add their own operators, too

[myguidingstar]

Should be implemented with some multimethods:
(operator-keyword? :custom-keyword) => true (default to false)
(parameter-count :custom-keyword) => number? or :any

(condition-statement "`table`.`column`" :custom-keyword [:arg-1 :arg-2])

=>

 ["`table`.`column` MY-OPERATOR (?, ?)" [:arg-1 :arg-2]]