Fix retain labels.

This is a numeric value based on the character offset of -k from the beginning of the switches.

src/types/lesser_element.rs

@@ -36,13 +36,15 @@ pub struct CommentBlock<'s> {
     pub contents: &'s str,
 }
 
+pub type CharOffsetInLine = u16;
+
 #[derive(Debug)]
 pub struct ExampleBlock<'s> {
     pub source: &'s str,
     pub name: &'s str,
     pub switches: Option<&'s str>,
     pub number_lines: Option<SwitchNumberLines>,
-    pub retain_labels: bool,
+    pub retain_labels: Option<CharOffsetInLine>,
     pub use_labels: bool,
     pub label_format: Option<&'s str>,
     pub contents: String,

src/types/mod.rs

@@ -37,6 +37,7 @@ pub use greater_element::Table;
 pub use greater_element::TableRow;
 pub use greater_element::TableRowType;
 pub use lesser_element::BabelCall;
+pub use lesser_element::CharOffsetInLine;
 pub use lesser_element::Clock;
 pub use lesser_element::Comment;
 pub use lesser_element::CommentBlock;