Add support for custom operators

[galves]

The idea here is to support custom RSQL operators, so we can express predicates like 'where location within "Ontario"'.

It has been achieved by casting the string operator to type Operator, which is less than ideal so please feel free to re-implement as you see fit. A more correct solution would be to make Operator into a class, but I wanted to avoid too much refactoring.

Thanks for the great library!

[coveralls]

Pull Request Test Coverage Report for Build 31

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 30:	0.0%
Covered Lines:	266
Relevant Lines:	266

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💛 - Coveralls

[tylerhubert]

Please check out this other Pull Request I just put up. https://github.com/tylerhubert/rsql-criteria-typescript/pull/10

I'm not so good at github, so not sure how to do the links. I built on top of what you have here and have provided an interface to create your own custom operators. This way you can create a single class that will have the operation string in it and not need to define a constant or the same string all over the place.

Please let me know if this will work!

[galves]

Perfect thanks!

[tylerhubert]

FYI, Version 2.1.0 is now available on NPM and has this change in it.

Thank you very much for the contribution!