Prioritized list of next 20 commands to implement

3. [ ] ls (not implemented, but there's `dir [-a|-A] [-p] [FILE...]`)
1. [x] tail
9. [ ] cp
4. [ ] ps -- HERE BE DRAGONS
10. [x] rm
19. [ ] chmod
2. [x] head
5. [x] env
6. [x] id
7. [x] mkdir
8. [ ] mv (in progress, #21)
11. [x] rmdir
12. [x] which
13. [ ] more
14. [ ] wc
15. [ ] who
16. [ ] seq
17. [ ] cut
18. [ ] uniq
20. [ ] chown