Add ability to set consumer names

When I look at the different consumers I've authorized using twurl, it's hard to tell which is which:

```
$ twurl accounts

greenberg
  8TM35KfwdyL4JZa3X8gGC (default)
  gkwkGnTLoX5f26vyTr9et
  EB9SWuaMGW3sb6xgNUSAH
```

This changes lets users associate a name with each consumer key. This name is displayed in the list of accounts:

```
$ bin/twurl accounts

greenberg
  8TM35KfwdyL4JZa3X8gGC - DebugApplication (default)
  gkwkGnTLoX5f26vyTr9et - ProductionApplication
  EB9SWuaMGW3sb6xgNUSAH - CoffeeShopsTweeter
```

A subsequent change could allow users to provide consumer names when changing the configuration via `twurl set default`.