Add support for Semicontinuous

[blegat]

It might be as simple as adding a line here:
https://github.com/JuliaOpt/CPLEX.jl/blob/master/src/MOI_wrapper.jl#L21
similar to
https://github.com/JuliaOpt/Gurobi.jl/blob/master/src/MOI_wrapper.jl#L28-L29