While usernames were now picked up correctly as a fallback from the
filename if none were present in the fields themselves, if there were
any in the fields the program would return an empty string.
This fixes it for both cases, the username existing in the entry fields
or only in the filename. Entry fields will still take precedence.
Will look for the username as before within the metadata of the entry,
but if there is no field which satisfies the conditions (by default
being named username, user, or login), will fall back to using the entry
name in pass (i.e. the filename).
fixes#3
