Use pluralization to create error messages

[krlmlr]

I love the new pluralization feature. To use it in tibble, I need to access this functionality for throwing or constructing error messages. Would you consider exporting glue_cmd(), perhaps with a different name and with an option to turn off markup?

[gaborcsardi]

We want to support formatting and styling error messages in cli, but it needs a bit more thinking.

[gaborcsardi]

The pluralization will be possible with #155. We still need to work out how exactly errors messages would use cli.

[gaborcsardi]

This is probably solved by the format_error() and cli_abort() functions.