The original implementation used str.encode() on input and str.decode() on
output. However this would cause UnicodeDecodeError since certain characters
can't be encoded / decoded in ASCII.

The new solution is to use unicode() on all input strings and output UTF-8
encoded strings. This makes the assumption that the shell can handle UTF-8
strings.

The original implementation used str.encode() on input and str.decode() on
output. However this would cause UnicodeDecodeError since certain characters
can't be encoded / decoded in ASCII.

The new solution is to use unicode() on all input strings and output UTF-8
encoded strings. This makes the assumption that the shell can handle UTF-8
strings.

Closes #242.

added a new option, -d/--decrease to decrease the weight of a path.
added --increase as an alias to --add for symetry.

The migration code was never working to begin with. Users migrating from v17 or
older will be starting from an empty, new database.

Also added unit test coverage to check database initialization, saving, and
loading.  Unit tests also revealed that migration code was not working properly
(starts database from scratch instead of copying existing entries over).