Redesign tibble formatting

[krlmlr]

with the following goals:

• extensibility
• clear responsibility between tibble, dplyr, and others
• separate printing/formatting of individual components (header, footer, ...)
• separation of the logic for formatting the columns
• support for format()
• configurable formatting for the body (and perhaps also header/footer)
• format() and print(): full support for Unicode everywhere
• glimpse(): full support for Unicode everywhere
• support for both dplyr 0.5.0. and dplyr 0.6.0

Something like:

1. We implement print() by forwarding to format()
2. format.tbl_df() calls format_head(), format_body() and format_extra(), default implementations are provided for tibbles
   • we cannot rely on format.data.frame() because it returns a data frame and not a character vector
3. dplyr needs to implement only format_head() for grouped data frames, and can append its information to the result of NextMethod()

The current implementation is inflexible because it relies on format.data.frame(). Therefore, this issue is mostly about replacing format.data.frame() with our own implementation.

We should release just before the dplyr release. I'll be closing a few related issues with a reference to this new issue.

@hadley: Could you please comment?

[hadley]

That design sounds good.

Maybe format_tail() for consistency? Or format_header() and format_footer()?