Added the `ppartial` function

`ppartial` is the same as `partial` except that its second argument
specifies a named argument to bind to when invoking the partially
evaluated function. Usage:

    ppartial(seq, length.out, from = 1, to = 2)(5)
    # [1] 1.00 1.25 1.50 1.75 2.00

It also defines the shortcut `pp`.
Merge branch 'ppartial'