Window navigation is reverted to vim's stock key combination, since tmux
is configured to work very similarly (using its prefix instead of
CTRL-W).
Window resizing is done more deterministically by adding/removing
columns/lines instead of using multipliers on the window height/width.
autocmd is used to hard code the file type to certain file names, since
vim expects them to have the standard names starting with a dot but the
files are tracked in the repository without the leading dot.
Add a <leader> mapping to quickly toggle between the current and
previous file.
Add a <leader> mapping to quickly toggle dark/light background.
Cosmetic change: all key codes are lower case.
