Update `install.sh` to honor `WALDO_USER_TOKEN` environment variable

[eBardX]

This invocation of install.sh should automatically call the new waldo auth command:

WALDO_USER_TOKEN=u-abcdef1234 curl -fLs https://github.com/waldoapp/waldo-go-cli/raw/master/install.sh | bash

If waldo auth fails, installation should still succeed, but an appropriate message should be displayed informing the user how to obtain a user token, and that running waldo auth is required before any(?) other Waldo CLI command will succeed.

If WALDO_USER_TOKEN is not set when install.sh is run, the same informational message should be displayed.