Add "na.print" as a new argument to "print.data.table"

[joshhwuu]

Closes #3152

• Confirm we want to honor na.print by exposing it as a new argument to print.data.table, instead of handling it through ... like print.data.frame.
• Discuss whether we want to handle error throwing/type checking within print.data.table or allow print.default to handle it.
• Add additional tests

Currently, na.print allows for either NULL or a character string to replace NA values when printing, but print.data.table does not handle this argument. I am suggesting we add the na.print argument to print.data.table and pass it to existing print.default calls, although I think it is possible to handle na.print through ..., although we will have to change some documentation and an extra check.

As for error throwing and type checking, I think the print.default error is clear enough:

invalid 'na.print' specification

However if we want to handle this error through print.data.table (maybe for clarity, translation, etc.), it should be easy enough to add. Currently quote is handled by print.default so I think doing the same for na.print would make sense.

[MichaelChirico]

Implementation looks good, including yes to including it in the print.data.table signature and to letting print.default to error handling. I think we just need NEWS.