I ran into a few issues using the Basics of Authentication sample app (advanced_server.rb):

1. The GitHub URL wasn't configurable. This is generally okay, but I needed to point the sample app at a different URL for testing.
2. I didn't like having to remember environment variables. 😁
3. Missing or mis-typed configuration options didn't fail loudly, and I usually found out about them by clicking around in the browser getting confusing error messages.

This PR introduces STDIN fallbacks for missing configuration. It also elevates the GitHub URL to a configuration option (defaulting to https://github.com instead of hardcoding in various places. The API base URL is set programmatically based on the URL (please check my logic here).

Happy-path usage:

ruby advanced_server.rb
Please enter a value for Client ID: 123
Please enter a value for Client Secret: 456
Please enter a value for Base GitHub URL or, just hit Enter to stick with the default (https://github.com):
<--------------------------------------------------------------------->
GitHub lives here: https://github.com
The GitHub API lives here: https://api.github.com
<--------------------------------------------------------------------->
[2015-12-07 16:16:42] INFO  WEBrick 1.3.1
[2015-12-07 16:16:42] INFO  ruby 2.1.5 (2014-11-13) [x86_64-darwin14.0]
== Sinatra/1.3.5 has taken the stage on 4567 for development with backup from WEBrick
[2015-12-07 16:16:42] INFO  WEBrick::HTTPServer#start: pid=57700 port=4567

Non-interactive usage:

GITHUB_URL=https://github.com \
  GITHUB_CLIENT_ID=123 \
  GITHUB_CLIENT_SECRET=456 \
  ruby advanced_server.rb
<--------------------------------------------------------------------->
GitHub lives here: https://github.com
The GitHub API lives here: https://api.github.com
<--------------------------------------------------------------------->
[2015-12-08 09:43:22] INFO  WEBrick 1.3.1
[2015-12-08 09:43:22] INFO  ruby 2.1.5 (2014-11-13) [x86_64-darwin14.0]
== Sinatra/1.3.5 has taken the stage on 4567 for development with backup from WEBrick
[2015-12-08 09:43:22] INFO  WEBrick::HTTPServer#start: pid=63796 port=4567

Specifying an alternate GitHub URL:

ruby advanced_server.rb
Please enter a value for Client ID: 123
Please enter a value for Client Secret: 456
Please enter a value for Base GitHub URL or, just hit Enter to stick with the default (https://github.com): https://ghe.acme.co
<--------------------------------------------------------------------->
GitHub lives here: https://ghe.acme.co
The GitHub API lives here: https://ghe.acme.co/api/v3
<--------------------------------------------------------------------->

Omitting a required option:

ruby advanced_server.rb
Please enter a value for Client ID:
Sorry, Client ID is required.