v0.6 Handle Older Unicode Versions

Demicode won't crash when ingesting UCD files from Unicode versions before 13.0.0 any more. The lack of some information and the presence of outdated property values are now gracefully ignored.

This release also changes how unassigned code points and sequences of more than one grapheme cluster are handled. Assuming that they may just be valid for some future version of the Unicode standard than the currently active one, demicode now elides blots for them and adds an explanatory note instead of the (non-existent) name.