ft-sdk-0.2.0

• BREAKING: renamed ft_sdk::Mountpoint -> ft_sdk::AppUrl<KEY="current-app">,
  the inner value is now an Option<String> instead of String.
• Added ft_sdk::AppUrl::{join, is_set}()
• Added ft_sdk::Result<T> alias to Result<T, ft_sdk::Error>.
• Added ft_sdk::Scheme extractor.

Full Changelog: ft-sdk-0.1.17...ft-sdk-0.2.0