Add :apply option to IO.inspect/3

[mgartner]

This is an attempt to build functionality of IO.inspect/3 that was mentioned in the proposal here: https://groups.google.com/forum/#!topic/elixir-lang-core/TUkmNHI4IbI

The goal is to make something like this...

["thing1", "thing2"]
|> generate_more_things()
|> (fn things ->
  things |> length() |> IO.inspect()
  things
end).()
|> do_something_with_things()

simpler to write, with something like this...

["thing1", "thing2"]
|> generate_more_things()
|> IO.inspect(apply: &length/1)
|> do_something_with_things()

[josevalim]

Thanks @mgartner! There is no agreement yet if the feature will be added and, if so, in which shape, so I will go ahead and close this until there is a conclusion. If we agree this is the way to go, then we will reopen it, review it and merge it!