Vim settings and timeouts in tmux config

Pressing ESC to return to normal mode in Vim has about a 1 second delay.
To fix it, add a timeout value of 0 to the TMUX config, and set a low
ttimeout value inside Vim.

In non-tmux terminals it may also be necessary to set KEYTIMEOUT=1
(value is timeout in milliseconds)
Update Vim sippets
Add the indispensible 'dunnet' alias
vimrc tweaks
Update vimrc

- Use ignorecase + smartcase
- Map both `jk` and `kj` to `ESC` (just mash both, doesn't matter which
  hits first)
- Don't try to map `C-m` to Normal mode add line below, because it causes
  the Return key to do weird stuff.  I don't know why.  Also use `I` for
  Normal mode add line above instead of `M`, to get back the standard
  `M` functionality of moving the cursor to the middle of the screen.