Ignore insecure directories on non-admin Zsh user

Commit c496edb updated the Zsh completion initialization script. This
resulted in an error for non-admin users:

```
zsh compinit: insecure directories and files, run compaudit for list.
```

I have admin and non-admin user accounts on my macOS work machine. For
security purposes (supposedly), I have to do my day-to-day-work with the
non-admin account, and only use the admin account for tasks requiring
those perms, like software upgrades. This means my Zsh installation, and
all related directories, are owned by the admin account, and I can't
simply `chmod` and `chown` to the non-admin account.

The solution is to ignore insecure directories on the non-admin account.

~/.zshrc:

```zsh
autoload -Uz compinit
if [ "$(whoami)" = "brendon.smith" ]; then
  compinit -i # Ignore insecure directories
else
  compinit
fi
```

The non-admin account doesn't have permissions to perform any actions on
the pertinent directories, so ignoring them doesn't pose a security risk
that I'm aware of.

Also see zsh-users/zsh-completions#680.

.zshrc

@@ -46,6 +46,10 @@ prompt pure
 if type brew &>/dev/null; then
   FPATH=$(brew --prefix)/share/zsh-completions:$FPATH
   autoload -Uz compinit
-  compinit
+  if [ "$(whoami)" = "brendon.smith" ]; then
+    compinit -i # Ignore insecure directories (perms issues for non-admin user)
+  else
+    compinit
+  fi
 fi
 source /usr/local/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh