Make using the `@vim$TMUX_PANE` window flag much more robust by
unsetting it every time the zsh prompt is drawn and setting it again
using a `FocusGained` `autocmd` in vim. This removes the need for the
hacky check for zsh in the conditional defined in the `in_vim` variable.
Previously the `autocmd` to replace the date on the first line if `Last
change: ` was present resulted in the cursor being moved in non `help`
filetypes due to the substitution failing. Now the substitution is only
attempted if the current file is a `help` file.
Add `autocmd` to automatically expand the `_template` snippet on the
`BufNewFile` event, if the expansion fails because the `_template`
snippet does not exist for the current `filetype` undo the changes. Do
the same thing when the `filetype` changes to `help`, since these are
`text` files to begin with the first `autocmd` has no effect.
