improve repr_text for tbl_df/data.table

[jankatins]

print.tbl_df() is optimized for "informative" output and currently we don't use it but coerce to an old fashioned data.frame in https://github.com/IRkernel/repr/blob/master/R/repr_matrix_df.r#L222

I like that we make the normal data.frame output more useable by including the ellipsis stuff, but IMO for tbl_df we are making it worse :-(

IMO we should also use the normal print.data.table stuff as that also shortens the df to 100 rows.

[
I'm currently using

repr_html.tbl_df <- function(obj, ...) NULL
repr_latex.tbl_df <- function(obj, ...) NULL
repr_text.tbl_df <- repr:::repr_text.default

to get useful output for my JSON stuff... :-(
]

[flying-sheep]

plain text is a step back though, so maybe we should just specifically support them