sage

• Use __STACKTRACE__ instead of deprecated System.stacktrace() function;
• Do not crash when inspecting sagas with non-atom stage names (thanks to @peaceful-james);
• [Allows users to pass in their own Supervisor for async steps](...

• Remove failed step name from compensation functions. This is done to purity compensations so that they don't carry too much information about the pipeline in which they run.
• Refactor how Sage mutates its state.
• Fix warnings from new Elixir versions and the new way how stacktraces are received. This bumps minimal Elixir version to 1.7.
• Bug fixes and documentation changes.

Fixed bug where retrying would erase effects created before the stage #53