Add parentheses in glimpse() for lists

[krlmlr]

> data_frame(a=list(1, 2:3)) %>% glimpse
Observations: 2
Variables: 1
$ a (list) 1, 2, 3

Should be:

$ a (list) 1, (2, 3)

[hadley]

Also that should probably be $a <list> to be consistent with the other views

[krlmlr]

Still need to surround type with <>, and add explicit tests for format_v.list().