Version 1.2.1
* Fixed a bug where a to-do item with no description would cause
  exporting the data to crash.
* Bumped the target SDK version to 23 (Marshmallow).
* Added a new preference item for vibrating when an alarm occurs.
* Fixed an old issue where preferences could not be changed
  when importing a file.
* When exporting or importing a file *and* on Marshmallow or higher,
  check whether the file location is outside of package-specific
  storage and if so request permission to access the folder if we
  don't already have it.
