Feature: Add depth setting and locality to VimwikiGenerateLinks

[rixx]

VimwikiGenerateLinks is useful for smaller directories, but less so for the root or a huge nested structure.

I'd like to be able to use it to

• Make an index 1 or 2 directories deep, not including all files, and
• Make an index of only the directory the file is in. My standard use case is to generate an index for a subdirectory, and (afaict), currently I have to manually enter that subdirectory as argument to VimwikiGenerateLinks. It would be nice to just do that automatically.