Read defaults from ~/.f5-cli.ini

Read credentials and default host from a `~/.f5-cli.ini` that looks like this:

```ini
[defaults]
host = my_f5

[mt-lb]
user = joe
password = secret
```

With this, you can leave out the `--host`, `--user`, and `--password` options:

```
$ f5-cli pool list
```

**Note: This builds off https://github.com/jdubs/f5-cli/pull/3**