add drake_deps()

[MilesMcBain]

Detects suggested packages required to use optional serialisation formats.

drake_plan(x = target(..., format = "fst")) creates a format column in the output plan which contains an "fst" element for the corresponding target.

[wlandau]

NB in development drake, make() and drake_config() now also have a format argument for those who want to set a global default format for all targets. Going to CRAN within the next couple weeks.