- Support for running a before_script: before starting tmux session.
- Separate run_before_script into its wrapper with custom exception.
- Create temp_session context manager.
- Kill session if before_script fails. Handle case with temp_session context mgr where session is killed within its scope.
- Update CHANGES.
- Add some docs for new before_script command
Merge pull request #71 from tony/OnlyIfStart

Support for running a before_script: before starting tmux session.