Use `.S3method()` rather than `registerS3method()`

[fkohrt]

The cookbook currently encourages the use of registerS3method(). The documentation at help("registerS3method", "base") says, however, that it is “not intended to be called directly”. As an alternative, the method .S3method() can be used (see also Kurt Hornik: S3 Method Lookup).

rmarkdown-cookbook/examples/print-method.Rmd

Lines 13 to 16 in ac81dd0

	registerS3method(
	"knit_print", "data.frame", knit_print.data.frame,
	envir = asNamespace("knitr")
	)

See also #186