Add offset to get the offset term from a model (#15)

Packages like GLM, MixedModels, and (at least theoretically)
BetaRegression support offsets for the linear predictor. It would be
nice to be able to query that in a unified way. This commit adds a
function `offset` that can be extended accordingly.

src/regressionmodel.jl

@@ -109,3 +109,11 @@ function reconstruct end
 In-place version of [`reconstruct`](@ref).
 """
 function reconstruct! end
+
+"""
+    offset(model::RegressionModel)
+
+Return the offset used in the model, i.e. the term added to the linear predictor with
+known coefficient 1.
+"""
+function offset end