Feature: generate smart Zettelkasten Ids and indexes

Currently, the Zettelkasten IDs are generated based on the creation time of the note. A smarter way would be to generate them to represent a logical hierarchy of the notes driven by folder hierarchy (in where the enex files are placed).

Example:
Let's assume that we have 3 notebooks in Evernote: Goals with notes titled g1, g2, DailyNotes with notes titled dn1, dn2 and dn3, and Projects with notes titled project1 and project2.
Goals and DailyNotes are in the same NotebookStack called Journal.

Let's assume that the working dir is Notes. Put projects.enex to it and create a folder called Journal. Then put goals.enex and dailynotes.enex to Journal. Hence we created a folder-representation of Notebook Stacks of Evernote.

The smart Zettelkasten Ids would be generated as follows:
> Journal -> 1
> > dailynotes -> 1.1
> > > dn1 -> 1.1.1
> > > dn2 -> 1.1.2
> > > dn3 -> 1.1.3

> > goals -> 1.2
> > > g1 -> 1.2.1
> > > g2 -> 1.2.2

> project.enex -> 2
> > project1 -> 2.1
> > project2 -> 2.2