Silence compiler warnings, safer variable re-binding

[tompave]

At the moment using this library in a phoenix project will raise this warning on (re)compilation:

warning: the variable "children" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

Unsafe variable found at:
  lib/my_app_name/react_io.ex:2

where lib/my_app_name/react_io.ex has these contents:

defmodule MyAppName.ReactIo do  
  use StdJsonIo, otp_app: :my_app_name, script: "node_modules/react-stdio/bin/react-stdio"
end 

This is because of Elixir's new stricter checks on variable re-binding inside conditionals.

This PR extracts the logic to add the reloader spec in a separate function, to improve the organization of the init/1 function and to silence the "unsafe variable assignment" warnings.

[gogocurtis]

+1

[tompave]

bump