some fixes on iex-dbg

* Breakpoints working, passing filename around when compiling
  files through `dc/2`.
* Handling parse errors on `dc/2`.
* Colors on some messages.
Helpers on debug shell shouldn't spawn a new process

* Helpers should be executed on the same process as the shell, as
  they may rely on the process dictionary.
working and callable IEx.Debugger.Helper.ds/3

* shells into a paused process.
* a little workaround enables the helper to know the pid of the
  server, but it's quite an ugly hack.