Added configurable options to the gem

Hi,

I've added configurable options to the gem so that you can customize some parameters.

Example:
```ruby
ApiTaster.configure do |config|
  config.title = 'MyTitle'
  config.call_to_action = 'My call to action text!'
  config.app_title = 'MyAppTitle'
  config.app_url = 'http://www.myapp.com'
  config.include_missing_definitions = false
  config.include_obsolete_definitions = false
end
```

By default, the gem is using these values:
```ruby
config.title = 'API Taster'
config.call_to_action = 'Select an API endpoint on the left to get started. :)'
config.app_title = 'Application'
config.app_url = '/'
config.include_missing_definitions = true
config.include_obsolete_definitions = true
```

Feel free to merge it if you want.