Handle digraph casing

src/main.rs

@@ -50,10 +50,10 @@ fn to_runic(latin: String) -> String {
   let mut chars = latin.chars().peekable();
   let mut out = String::with_capacity(latin.len());
   while let Some(c) = chars.next() {
-    if let Some(digraphs) = DIGRAPHS.get(&c) {
+    if let Some(digraphs) = DIGRAPHS.get(&c.to_ascii_lowercase()) {
       if let Some((_, rune)) = digraphs.iter().find(|(second, _)| match chars.peek() {
-        Some(peeked) => second == peeked,  // unstable: chars.peek().contains(&second)
-        None => false,
+        Some(peeked) => second == &peeked.to_ascii_lowercase(),
+        None => false,  // unstable: chars.peek().contains(&second.to_ascii_lowercase())
       }) {
        chars.next();
         out.push(*rune);