More exa support
More exa support

- Set `EXA_TREE_IGNORE` if the user doesn't already have it set up
- Add `tree` alias to use `exa` if there isn't already a
  system-installed `tree` command.

Signed-off-by: Joe Block <13bb081c6ac072ef78a3e348068e2204400d7335@unixorn.net>

unixorn/zsh-quickstart-kit