text model objects need extractor methods

[kbenoit]

textmodel_fitted_* and textmodel_predicted_* objects need more extractor and other methods, similar to those for lm class objects. Currently, just print and predict methods are implemented. Candidates are:

• confint
• formula
• summary
• model.frame and `model.matrix
• ndoc, feature
• plot
• simulate?
• coef

[koheiw]

See #1007 too.

[kbenoit]

Now being resolved as part of #1007.