Based on #21 and also OpenTTD/bananas-api#88

• Change: Follow redirect convention for login also for developer flow bananas-api#88 introduced the same redirect flow for developer login as we have for github login.
• Fix: do not try to authenticate when requesting help (#4) #21 changed the login flow on the clientside to be handled in the actual commands instead of on startup. @TrueBrain did mention on IRC he is interested in authentication using decorators instead, but in my opinion that is not a priority for a one-liner.

In this PR I allow the user to set the oauth audience (previously hardcoded) to 'developer', and replace a hard-coded client id with the already configurable one. Finally, if the redirect url is a relative url (starting with /), the api-url is prepended.

Together with the changes on the bananas-api server side, this allows a user to use developer authentication.