add a member_of() function

[cjyetman]

add a member_of() function that takes a vector as the first argument, the name of a group or association as a second argument, and returns a logical vector the same length as the vector passed as the first argument. Possibly have a third argument that allows one to choose the country code type of the first input vector (e.g. 'iso3c', 'wb', etc.), but should default to 'country.name'. Internally it would use the countrycode package to match the input vector elements to elements in the memberstates list. Write it in a way that facilitates using it within a magrittr/dplyr chain (first argument accepts the input data and returns the modified version of the input data).