Support x[!!y] anti-anti-join syntax

[ctbrown]

Ref. http://stackoverflow.com/questions/18969420/perform-a-semi-join-with-data-table

Matt asked for a feature request for this in the stack overflow comments; I did not see this in the issues and the feature does not appear to be implemented as of 1.9.4. I am just adding this to ensure that this is not lost, in all likelihood this is low priority.

[gennaro-tedesco]

I would like to encourage this request as well, if possible, along the same lines of the equivalent dplyr syntax anti_join(first, second, by = 'value').

[jdthorpe]

See also #2279