allow for custom filter function

[skewballfox]

A while ago, I was working on a recursive version that included files that were linked from start at(#212 ). While I totally get that merging that functionality may not be desired, there is a more minor change that would definitely help when using obsidian export as a library.

Right now, the way start at works is that it's set equal to root unless declared and then start at is used to filter both the results and the generated paths. Something that would be when using this as a library and doing any multistage iteration would be to make the following changes.

• change the filter function to be like postprocessor, but a single function
• strip start at component of path as a postprocess

If this is a change you'd be comfortable with, I could open a PR.