Add ls command to experiment with directory exploration.
Implement the built-in path command.

- Added a linked list library of my design to implement the path
  list. This is modified from my snippet on github, using strings
  instead of ints. I also modified it to work with the error handling
  present in the rest of the project.
- Added the "path" command, which allows you to add, remove, and print
  the items on the internal PATH variable.
- Changed the makefile std tag to gnu99, which allows for strdup().
- Added a "test" build in the makefile, which does a sanity check
  on the functioning of all built-in commands.