approved tokens list
I find it too painful to pass `--token ...` every time to `./swapcli balances`. I find myself having to lookup the token address every time and I even forget which tokens I have. I'm going to propose a possible solution, but anyone who thinks they have a better idea please add commentary.

Solution: We have a tokens.yml file in `${DATA_DIR}/${ENV}`.  I'm picking YAML instead of JSON so users can add comments. In the file is a list of addresses of tokens that an individual user has verified. These tokens are always shown by the `balances` subcommand to `swapcli` and an entry is required if a user wants to take an offer for that token. This will prevent users from taking a token that has a deceptive symbol name, say USDT, but isn't at the official USDT address.

Open questions: Would the user always edit this file by hand, or would we have a `swapcli` subcommand to add an entry? If the file does not exist, we can always create a template version with no entries to help the user.