Apply drive_id class to the id variable in a dribble

[jennybc]

It just occurred to me that the id variable in a dribble could have type drive_id. We already have this class and we can apply it inside the functions we already have for creating dribbles. This makes it more attractive to write a format method for drive_id objects that truncates / ellipsizes the ids. This is basically a revival of #5.

[LucyMcGowan]

Working on this this morning - when do you think would be the most natural time to introduce this class for as_dribble.data.frame() -- if passed a list, it is natural to assign the id a class, but for data frames currently we pass it through a new_dribble() function (that doesn't check any columns, just assigns the class) and then dribble_validate(). My inclination is new_dribble(), but then we'd be semi-validating it before it's been passed to dribble_validate() (to at least make sure it has a column id and assign it class drive_id.

[jennybc]

I'm not sure when to apply the class. The drive_id formatting isn't going to work in the near future anyway (tidyverse/tibble#212), so let's wait and see what the world looks like once that's cleared up and we're motivated to move forward.