Features

This release primarily expands the functionality of prediction() and adds support for additional new model classes. Specifically:

prediction() methods gain an at argument (like in margins::margins()) so that predicted values can be calculated for counterfactual datasets with specified values of covariates. (#13)
prediction() methods for models of multilevel outcomes (e.g., ordered probit, etc.) gain a category argument to be used to dictate which level is expressed as the "fitted" column. Predicted probabilities for all levels are returned as named columns; this simply toggles which column is additionally included as the "fitted" column. (#14)

New model classes supported:

Added prediction.zeroinfl() method for "zeroinfl" objects from pscl. (#1)
Added prediction.hurdle() method for "hurdle" objects from pscl. (#1)
Added prediction.lme() method for "lme" and "nlme" objects from nlme. (#1)
Added prediction.mnp() method for "mnp" objects from MNP. (#1)
Added prediction.mnlogit() method for "mnlogit" objects from mnlogit. (#1)
Added prediction.gee() method for "gee" objects from gee. (#1)
Added prediction.lqs() method for "lqs" objects from MASS. (#1)
Added prediction.mca() method for "mca" objects from MASS. (#1)
Added prediction.plm() method for "plm" objects from plm. (#1)
Added prediction.princomp() method for "princomp" objects from stats. (#1)
Added prediction.ppr() method for "ppr" objects from stats. (#1)
Added prediction.naiveBayes() method for "naiveBayes" objects from e1071. (#1)
Added prediction.rlm() method for "rlm" objects from MASS. (#1)
Added prediction.qda() method for "qda" objects from MASS. (#1)
Added prediction.lda() method for "lda" objects from MASS. (#1)
Noted (built-in) support for "brglm" objects from brglm via the prediction.glm() method. (#1)
Documented prediction.merMod().

Other changes and bug fixes

find_data() now respects the subset argument in an original model call. (#15)
find_data() now respects the na.action argument in an original model call. (#15)
find_data() now gracefully fails when a model is specified without a formula. (#16)
prediction() methods no longer add a "fit" or "se.fit" class to any columns. Fitted values are identifiable by the column name only.
Made mean_or_mode() and median_or_mode() S3 generics and added .data.frame() methods for both.
Fixed a bug in mean_or_mode() and median_or_mode() where incorrect factor levels were being returned.
Expanded test suite considerably and updated CONTRIBUTING.md to reflect expected test-driven development.