feat(summary): add strategy which categorizes current directory only

[nuhakala]

Why

I like to structure my notes in hierarchial manner inside folders. However, the workspace summary creates a summary of the whole workspace, even though I would like to create a summary of only current directory, creating links to subfolder index files. And that is what these modifications do.

What

I added new strategy current_dir which creates links to subfolder index.norg files and categorizes the regular files in the directory as before. If subdirectory does not contain index.norg file, it is not created and the link is not added.

Open points / problems

• What I did, I just copied the default strategy and modified the beginning of it a little. However, this leads the module to have quite a lot of duplicated code.
• I also had to read the files of the current directory again, which is inefficient.

P.S. This is my first pull request to open source project ever. I hope the changes are not too bad.