Testcases:

- [x] trace01.txt - Properly terminate on EOF.
- [x] trace02.txt - Process builtin quit command.
- [x] trace03.txt - Run a foreground job.
- [ ] trace04.txt - Run a background job.
- [ ] trace05.txt - Process jobs builtin command.
- [ ] trace06.txt - Forward SIGINT to foreground job.
- [ ] trace07.txt - Forward SIGINT only to foreground job.
- [ ] trace08.txt - Forward SIGTSTP only to foreground job.
- [ ] trace09.txt - Process bg builtin command
- [ ] trace10.txt - Process fg builtin command. 
- [ ] trace11.txt - Forward SIGINT to every process in foreground process group
- [ ] trace12.txt - Forward SIGTSTP to every process in foreground process group
- [ ] trace13.txt - Restart every stopped process in process group
- [ ] trace14.txt - Simple error handling
- [ ] trace15.txt - Putting it all together