Allow async execution and cancelling - Session.signal Session.execute_async

[dbernheisel]

Following on #11 but with a different API and closer to real bash mannerisms.

• Add Bash.Session.signal(..., signal) to give more control on how to kill the bash session. If passing in the session, it's going to signal to the current execution. If passing in a reference, it will cancel that execution. If it hasn't executed yet (eg, there are multiple chained execute_async and it hasn't gotten there yet, it will pop from the queue and have a synthetic exit.
• Breaking Adjust Bash.execute_async to return a set of references for tracking execution, before it would simply return :ok but now it's {:ok, info}. This allows the user to cancel that execution.
• Multiple Bash.Session.execute_async on a session will queue up the executions serially. This somewhat acts like merging Bash scripts, but they're chunked and individually await-able.
• Not breaking, but Bash.execute now uses execute_async but awaits upon it.
• When cancelling an in-progress Bash session, depending on the signal, it will go from SIGTERM -> grace period -> SIGKILL. If a bash script has a trap, this is important so it has time to execute the trap. Folks will likely want this by default, though they're still welcome to signal a kill and skip the trap (just like in real bash).

@lostbean would you mind reviewing and testing this to see if this meets your needs?