Support for csvy in packages

[hadley]

This would require:

• Some advice/tooling for how to place directly in data/ (i.e. accompanied by script that called read_csvy())

• Some tool to turn metadata into a nice Rd file (via roxygen or otherwise)

[hadley]

A little experimentation suggests that .R scripts in data/ are run twice at install time, which doesn't seem super appealing.