This commit introduces a new module of harpoon, "harpoon.tmux", which
provides several functions for using tmux windows for harpoon terminals.
So far, all the basic features are supported, such as creating and
moving to a new terminal, deleting all terminals, and sending commands
to terminals etc.
I had to normalize the paths due to the fact that sometimes a path to a
file would be absolute or relative to the project depending on how you
opened up the file. To prevent this from creating duplicates in a mark
list, I normalized the path before doing anything.
