[Fix] Keybinding conflict introduced as part of #526.

Here's what I've done to test this fix:

- Fire up `vim` only (outside of `tmux`), create several splits. Verify that I am able to use `<c-{h,j,k,l}>` to move between them.
- Fire up `tmux` with two ***tmux*** splits.
    1. In one of them (doesn't matter which), open up `vim` and create some ***vim*** splits.
    2. Verify that I can move between both `tmux` and `vim` splits with the same `<c-{h,j,k,l}>` keys.
        - With cursor focus in `vim`, if I hit `<c-\>`, verify that the NERDTree panel is opened as expected.
        - With cursor focus in `tmux`, if I hit `<c-\>`, verify that the last active ***tmux*** split is switched to.

Finally, what I *haven't* tested are vim splits within GVim/MacVim. Outside of that, pending some sanity checks (cc: @skwp), I think this should be good to go functionality-wise.