_3llo: init at 0.3.0

Simple CLI client for `trello.com`. It can be used like this:

```
$ export TRELLO_USER=your_username
$ export TRELLO_KEY=your_key
$ export TRELLO_TOKEN=your_token
$ ./result/bin/3llo
```

I didn't create a module for this as I don't think that those secrets
should live in the Nix store. Ideally `3llo` can be used from a script
which retrieves secrets from some kind of password store like this:

```
export TRELLO_KEY=$(pass show trello/key)
export TRELLO_TOKEN=$(pass show trello/token)
3llo $@
```