[Feature request]: Authorization flow for "web app"

[bevzzz]

Reddit API allows authorization for 3 app types:

• web app
• installed app
• script

At the moment it seems that go-reddit only supports the latter use case -- oauthTransport uses Username + Password to obtain and renew access tokens.

In order to send requests to Reddit's API on behalf of multiple users, we need to support OAuth2 for "web apps". (see also the API docs). Should be a rather simple change, as the standard oauth2 library also supports exchanging code for token.

I am using go-reddit in one of my private projects and need this functionality, so I'll be happy to contribute a PR :)