fix(summary): ordering and indentation of nested entries

[nuhakala]

Current state sometimes mixes entries with nested categories. For example, if there is three items:

• C with category cat
• A with category cat.sub
• B with category cat.sub

The produced summary looks like this:

* Index
** Cat
*** Sub
   - A
   - B
   - C

even though I want C to be directly under Cat. With multiple notes this causes nested categories to be rather messy sometimes.

I fixed this by simply introducing few temporary variables and adding nested categories to the summary at the end.

[vhyrro]

Code looks good! I'll also play around with the implementation later today to make sure it all works :)

[vhyrro]

Seems to work well for me, thanks for the PR!