let username and api_key be a subset of creds

Now, saving username and api_key won’t overwrite other fields that may
exist in ~/.plotly/.credentials. Also, you can save more key-value pair
credentials to ~/.plotly/.credentials with:

```
saveplotlycredentials(‘example_username’, ‘example_api_key’,
struct(‘extra_key’, ‘extra_value’))
```
let the url endpoint be a config variable
Merge pull request #6 from plotly/variable_server_endpoint

Variable server endpoint