Allow ident containing number

Fixes #34.
diff --git a/impl/src/fmt.rs b/impl/src/fmt.rs
index be897e5..9672f4f 100644
--- a/impl/src/fmt.rs
+++ b/impl/src/fmt.rs
@@ -63,7 +63,7 @@
     let mut ident = String::new();
     for (i, ch) in read.char_indices() {
         match ch {
-            'a'..='z' | 'A'..='Z' | '_' => ident.push(ch),
+            'a'..='z' | 'A'..='Z' | '0'..='9' | '_' => ident.push(ch),
             _ => {
                 *read = &read[i..];
                 break;