Add support for non-recursive version of list_modify

[vspinu]

Sometimes recursion is not needed when modifying lists. Would be nice to have recursive = TRUE argument to list_modify and list_merge. Or maybe a bit more control over which objects should be considered atomic. For instance, one often it's useful to treat data.frames as atomic.

> a <- list(x = data.frame(one = 1, two = 2))
> b <- list(x = data.frame(three = 3))
> list_modify(a, b)
# current
$x
  one two three
1   1   2     3
# desired
$x
  three
1     3

[hadley]

Agreed, I think this is what I usually want.