Make `var_names` accessible in `model.fit()`

[digicosmos86]

In PyMC, pm.Sample() has a var_names parameter which allows the users to specify the variable names to be included in InferenceData. In Bambi, it seems that _run_mcmc() will override this argument when calling pm.sample(), leaving the users with no option to specify the variables that they want to include in the InferenceData. Maybe var_names can be added to model.fit() to make it possible to pass the user's own var_names?