Add optional pin/fingerprint protection

[derekantrican]

Simplenote (and I'm sure - others) has this feature. Whenever the app is opened (or device is unlocked while app is open, or app is switched to from multitasking) it will require a pin (or, optionally, a fingerprint) in order to access the notes. Based on the user's preference, obviously

[kohei-takata]

Thanks @derekantrican ! So it for security? I'm not sure how many people want this feature. So I will not develop this feature sooner. PRs are welcome!

[derekantrican]

Yeah, for security. Of course - it's definitely a "down-the-line" feature