Add surrounding function name delete/change

Adds `f`/`F` support to `s:dosurround`, to allow changing/deleting the name of a surrounding function call. As with `t`, `f` preserves whitespace while `F` removes it. For example,

    Old text                  Command      New text
    func(arg1, ar*g2)         dsf          arg1, arg2
    func(  arg1, ar*g2  )     csf)         (  arg1, arg2  )
    func(  arg1, ar*g2  )     csF)         (arg1, arg2)