feat: add downloads directory argument for attachment download

Refs: #559

src/lib.rs

@@ -6,5 +6,19 @@ pub mod email;
 pub mod folder;
 pub mod manual;
 
+use std::path::PathBuf;
+
+use shellexpand_utils::{canonicalize, expand};
+
 #[doc(inline)]
 pub use crate::email::{envelope, flag, message};
+
+/// Parse the given [`str`] as [`PathBuf`].
+///
+/// The path is first shell expanded, then canonicalized (if
+/// applicable).
+fn dir_parser(path: &str) -> Result<PathBuf, String> {
+    expand::try_path(path)
+        .map(canonicalize::path)
+        .map_err(|err| err.to_string())
+}