Implement QR code generator for share URLs

Allow users to generate and show a QR code in the terminal for a share URL after uploading.
Requested in: https://github.com/timvisee/ffsend/issues/19#issuecomment-472998495

• Find QR code crate: qr2term
• Add compiler feature flag: qr
• Implement --qr flag for uploads
• Print QR code in terminal