implement subset filter operator, update readme

[arifogel]

I added the subset filter operator.
You can do things like:
?(@.availableSizes subset ['S', 'M', 'L'])
or
?(['S', 'M', 'L'] subset @.availableSizes)
In order to check for proper subset, for example you could do:
?(@.availableSizes subset ['S', 'M', 'L'] && !(['S', 'M', 'L'] subset @.availableSizes))

[jochenberger]

Could you please add tests?
I wonder if there might be a better name for the operator than subset. At least to me, it wasn't clear about the operator's direction. Maybe subsetof?

[jochenberger]

This is (more or less) fixed via #365

[arifogel]

Closing since @dhalperi took care of this.