Pin to unlock

[crankycaleb]

I'm sure related to some of the ground work for #22
The way other password managers have handled it, whenever you reboot your phone you'll type in your full master password to unlock the archive initially. Then you'll just unlock it with a pin for subsequent launches until the next fresh boot of the device.
This should help cover devices without fingerprint readers, and also a good feature to have since the added security of a pin over fingerprints has been a hot topic lately (privacy).