Implement an optional URL shortener

Implement an optional URL shortener to simplify share URLs.
Requested in: https://github.com/timvisee/ffsend/issues/19

• Find URL shortener we can use: using is.gd for now: cca5889e
• Implement compiler feature flag for support: urlshorten: cca5889e
• Implement --short flag for uploading: cca5889e
• Support downloading from shortened URLs (follow URL redirections): dd3a4c56
• Show security warning by default when shortening, unless --yes or --force is given: e2ebbc91