Chg: Don't Box FilterFn in WalkOptions

Previously, `filter_fn` on the `WalkOptions` struct looked like:

    pub filter_fn: Option<Box<&'static FilterFn>>,

This boxing was unneccesary and has been changed to:

    pub filter_fn: Option<&'static FilterFn>,

This will only affect people who use obsidian-export as a library in
other Rust programs, not users of the CLI.

For those library users, they no longer need to supply `FilterFn`
wrapped in a Box.

src/walker.rs

@@ -28,7 +28,7 @@ pub struct WalkOptions<'a> {
     /// it should be included or not.
     ///
     /// This is passed to [`ignore::WalkBuilder::filter_entry`].
-    pub filter_fn: Option<Box<&'static FilterFn>>,
+    pub filter_fn: Option<&'static FilterFn>,
 }
 
 impl<'a> fmt::Debug for WalkOptions<'a> {