Add a dependency manager for render-time actions.

This will be used for supporting things like copying static files or rendering code blocks like gnuplot or graphviz.

src/context/dependency_manager.rs

@@ -0,0 +1,19 @@
+use std::path::PathBuf;
+
+pub(crate) type RefDependencyManager = std::sync::Arc<std::sync::Mutex<DependencyManager>>;
+
+#[derive(Debug)]
+pub(crate) struct DependencyManager {
+    /// A stack of paths for the files being visited.
+    ///
+    /// The last entry is the current file being processed. This can be used for handling relative-path links.
+    file_stack: Vec<PathBuf>,
+}
+
+impl DependencyManager {
+    pub(crate) fn new() -> Self {
+        DependencyManager {
+            file_stack: Vec::new(),
+        }
+    }
+}